Telektronikk 2/3.2001

1 Introduction

From the outset, one may say the Internet Proto-

col (IP) was intended to transport information

from a source to a destination. The main empha-

sis may be that the information eventually

arrived with less strict requirements on time.

This may work well for some applications, like

computers and sensors exchanging data, and

when no impatient human being is involved.

However, as more types of applications are

loaded on the IP-based networks, additional

requirements are also placed on these networks.

Hence, the question arose how to efficiently sup-

port these applications. This became even

stronger as the commercial concerns grew for

providing services on the IP-based networks.

This may be the main motivation for proposing

the mechanisms described in this paper. Moving

beyond the best effort, other service models were

introduced; the Integrated Services (IntServ) and

the Differentiated Services (DiffServ) models.

Protocols for reserving resources were also pro-

posed, including the Resource reSerVation Pro-

tocol (RSVP). Avoiding the routing processing

and introducing means for load distribution and

traffic flow protection, the MultiProtocol Label

Switching (MPLS) was described. These are

described in the following chapters. Some basic

packet handling mechanisms are outlined first

(Chapter 2).

Quite a lot of variations and detailed information

is available on these subjects, and references are

pointing at the sources. In addition to the huge

background material refinements are steadily

on-going.

2 Congestion and

Packet Handling

Congestionis said to arise when too many pack-

ets (too high a load) are present in a sub-network

compared to the capacity of that sub-network.

Congestion has a tendency to feed upon itself,

for example as packets queued up may be timed

out and retransmitted adding to the congestion.

Another phenomenon is the spreading as un-

acknowledged packets will be buffered and

do not release memory space.

In this chapter means for managing congestion

are described. Basically, they are addressing

how packets are handled in an end-system and

in a router. The flow/congestion control mecha-

nisms in TCP are essential, as described in

[Jens01].

Congestion avoidanceis simply to avoid the

occurrence of congestion. Congestion avoidance

constitutes preventive congestion management

policies that can be categorised as having a long,

medium or short response time scale. Long-term

policies include capacity planning to expand net-

work capacity using estimates or forecasts of

future traffic demand and distribution. Medium-

term policies cover the minutes to days time

scale. Examples are adjusting routing parameters

and reconfiguring the logical network topology.

Short-term congestion avoidance includes packet

level processing functions as returned to later in

this chapter.

Congestion controlis commonly used to make

sure that a sub-network is able to carry the

offered traffic efficiently, involving all traffic

flows passing. On the other hand, flow controlis

used between a pair of sender and receiver pre-

venting the sender from transmitting packets too

fast, involving feedback from the receiver to the

sender. However, such a feedback may also be

set by the network, e.g. to avoid congestion,

implying that flow control mechanisms can be

utilised related to congestion control.

Several mechanisms (called policies in [Tane96])

affect congestion and how it could be dealt with,

see Figure 1, including:

• At the end-to-end level: retransmission policy,
  out-of-order buffering policy, acknowledge-
  ment policy, flow control policy, timeout
  policy.


• At the network level: use of connections, pol-
  icy for queueing and serving packets, discard
  policy (buffer is full), routing policy, packet
  lifetime policy.

Basic IP-related Mechanisms

TERJE JENSEN

When reading about IP-based networks, one almost immediately bangs into a series of abbreviations

and expressions. The main objective of this paper is to outline the basic mechanisms; Best Effort, Differ-

entiated Services, Integrated Services, MultiProtocol Label Switching, Resource reSerVation Protocol,

Label Distribution Protocol, and some of the ways packets are buffered and scheduled in a router.

Terje Jensen (39) is Research

Manager at Telenor R&D,

Kjeller. He earned his PhD

degree in 1995 from the Norwe-

gian University of Science and

Technology. Activities include

performance modelling and

analysis, dimensioning and net-

work evolution studies.

[email protected]

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