Side_1_360

In this document CoS is used to group traffic on
the basis of performance related requirements
(quantitative or qualitative) like loss, delay,
delay variation, throughput and resilience and in
addition priority and elasticity (Transmission
Control Protocol (TCP) or User Datagram Proto-
col (UDP)). A service can consist of one or many
service components as illustrated in Figure 1.
One example can be a multi-media service with
a voice, a video and a data service component.
Each service component belongs to a CoS.
Finally, a mapping is presumed between CoS
and PHB group that is unique within a domain.

Lack of Control makes Service

Differentiation Difficult

In the following an example is given with traffic
offered to a DiffServ capable router (Cisco
7507). The traffic offered is constant UDP traffic
from a SmartBits tester. Although this is not a
realistic test scenario, since UDP does not have
the important feedback control of TCP and nor-
mal traffic variations are not present, some
important points are shown.

In the test scenario we have used four CoS and
Low Latency Queueing towards a POS STM1
interface. The CoS are:

CoS 1: Traffic with strict real-time requirement.
The service can be Voice over IP (VoIP). The
traffic is mapped to the Expedited Forwarding
PHB and uses a strict priority queue with rate
limit 23.5 Mbit/s (15 – 16 %). The packet size is
110 byte (IP).

CoS 2: Streaming traffic. This is non-priority
traffic with real-time requirements, but with
looser requirements than CoS 1. The traffic is
mapped to an Assured Forwarding PHB class
using WFQ with weight 50 %. The packet size
is 942 byte (IP).

CoS 3: Better than Best Effort (BBE) traffic or
Business Class. This is non-priority traffic with
no real-time requirement but high requirement
on loss and throughput. This traffic should be
based on a protocol like TCP. The traffic is
mapped to an Assured Forwarding PHB class
using WFQ with weight 25 %. The packet size
is 622 byte (IP).

CoS 4: Traffic with unspecified requirements
like today’s Internet (but will probably be given
higher throughput requirements than in today’s
network). This is BE traffic and uses WFQ with
weight 25 %. The packet size is 622 byte (IP).

Traffic from a given CoS is mapped to a unique
queue at the router output interface, and different
classes use different queues. While CoS 1 has
absolute delay priority over the other classes,

the other classes use Class Based Weighted Fair

Queuing (Figure 2). So these classed are served

according to pre-defined weights.

There is however one exception in this imple-

mentation (Cisco 7507) and this is the Tx-buffer.

Packets always go via a common buffer, the Tx-

buffer. Only when this buffer is full are incom-

ing packets forwarded to the Class Queues as

given in Figure 2.

In the example the load is increased linearly so

that the relative proportion between the classes

is kept. We observe that as the load increases the

different classes get their relative share of the

throughput as given by the scheduler (fair share).

Some classes get more as long as some of the

other classes do not use their reserved band-

width. VoIP starts to lose packets when offered

traffic reaches the rate limit configured.

The latency as a function of total offered traffic

is given in Figure 3.

In the given example the BE queue starts to

grow as soon as congestion state is reached. The

latency fast approaches a maximum value corre-

sponding to the buffer size for this queue. Due to

the Tx-buffer the latency increases accordingly

for all the other classes as soon as congestion

state occurs. In the example this increase is from

0.5 ms to 4–5 ms. With the given offered traffic

and configuration, VoIP is the next class to get

Figure 1 The relation between

service description, CoS and

DiffServ classes

Figure 2 Low latency queuing

Service

description

Service

component n

EF + back-up path

AF 1

CoS 1

Service

component i

Service

component 1

CoS m

EF class

RT AF class

NRT AF class

BE class

Link

Absolute priority (HOL) or

peak rate shaper combined

with delay priority

Class Based

Weighted Fair

Queueing