Side_1_360

As explained in [Jens01] DiffServ uses a partic-

ular implementation of the IP version 4 Type of

Service (ToS) header field. This field is now

called the DiffServ field, consisting of 8 bits, out

of which 6 bits are available for current use and

two are reserved for future use. The 6 bits define

the DiffServ Code Point(DSCP), which identi-

fies a Per-Hop Behaviour(PHB). The PHB indi-

cates the way packets shall be handled in the

routers and can be set and reset in any DiffServ

capable node, also referred to as marking the IP

packet. Some standardised PHBs are: default

class (DE, [RFC2474]), Class Selector (CS,

[RFC2474]), Expedited Forwarding (EF,

[RFC2598]), and Assured Forwarding (AF,

[RFC2597]).

EF is described as a forwarding treatment for

supporting a low loss, delay and jitter end-to-end

service with assured bandwidth. The exact way

to implement such a service by mechanisms in

the network is left open for the operators/ven-

dors.

AF is a group of PHBs. The idea is to support

services with requirements for assured packet

delivery. It is assumed that customers have sub-

scription-based traffic profiles, Service Level

Specification (SLS). Packets within the profile

shall have a high probability of delivery, where-

as out-of-profile packets can be delivered if

bandwidth is available in the network. For this

purpose packets are given different drop prece-

dences. The AF group defines four classes of

traffic. For each class three drop precedences

are defined, each representing a PHB and thus

a reserved DSCP value.

A central term in DiffServ is a Behaviour Aggre-

gate(BA). This is the aggregation of all packets

with the same DSCP and crossing a given link in

a particular direction [RFC2475]. The set of

BAs sharing an ordering constraint is called an

Ordered Aggregate(OA). For example, all pack-

ets belonging to a given AF class and crossing

a given link in a particular direction share an

ordering constraint. This is because the AF defi-

nition states that AF packets of the same micro-

flow belonging to the same AF class must not be

reordered (their sequence must not be changed)

regardless of their drop precedence.

Another term is the Per-Hop Behaviour Schedul-

ing Class(PSC). A PSC is the set of PHBs that

are applied to the BAs belonging to an OA. For

example, the PHB that is associated with a given

AF class constitute a PSC. Hence, PSC is a PHB

group for which a common constraint is that

ordering of at least those packets belonging to

the same microflow must be preserved.

A Service Level Specification(SLS) is defined

([ID_dsterms]) as a set of parameters and their

values which together define the service offered

to a traffic stream by a DS domain. An integral

element of an SLS is the Traffic Conditioning

Specification(TCS). The TCS is defined as a set

of parameters and their values which together

specify a set of classifier rules and a traffic pro-

file. These terms are illustrated in Figure 17.

The terms in the SLS are checked at the border

of the domain, e.g. in an edge router. In case the

appropriate DSCP value has not been inserted in

the packet, this has to be done, based on various

combinations of information in the packet. This

information may include the IP packet header as

well as the header of the transport protocol.

Additional information may also be used, like

the interface on which the packet arrived on.

This means that a Multi-Field(MF) classifier

and marker would be activated in the first Diff-

Serv-capable router in the domain. Then the

packet has got its DSCP implying that a BA is

given. Characteristics of a flow (aggregate) will

then be monitored to see whether the packet is

forwarded directly (conditions in the SLS are

obeyed), being dropped, re-marked or shaped.

A logical relation between the different func-

tions is illustrated in Figure 18.

A distinction is made between an MF classifier

and a BA classifier. The MF classifier classifies

packets based on one ore more fields in the

packet. This is normally only done at the edge of

the network. The BA classifier classifies packets

Figure 16 DiffServ uses
service classes
(traffic aggregates)

Figure 17 Some terms
related to DiffServ

Assignment to
queue based on DSCP

service per

class (PHB)

PHB = treatment of a BA in each hop (EF, AF, BE, ...)

PSC = set of PHBs applied

to BAs sharing an

ordering constraint

SLS
Service Level
Specification
TCS
Traffic Conditioning
Specification

Aggregation of packets in
Behavior Aggregates (BA)

OA = set of BAs sharing an

ordering constraint