Second International Conference on Electrical Engineering

25-26 March 2008

University of Engineering and Technology, Lahore (Pakistan)

978-1-4244-2293-7/08/$25.00 ©2008 IEEE.

MPLS Feasibility for General & Core IP Networks using Open-Source

Systems Syed Adeel Ali Shah (adeelbanuri@hotmail.com) Laeeq Ahmed (lahmed@nwfpuet.edu.pk)

Department of Computer Science and Information Technology

N-W.F.P University of Engineering and Technology, Peshawar, Pakistan.

Abstract Over the past few years Internet growth has

become rapid and is now considered as primary

source of information sharing and communication.

This has resulted in development of new

applications and communication systems that

demand more service quality, reliability and

efficiency. These applications not only include

traditional data but also new applications for

voice, video conferencing along with peer-to-peer

applications. Current IP protocol suit is although

the dominant networking technology, but despite

its dominance, it has been exposed with

weaknesses regarding ever-increasing demands by

Internet applications. This paper surveys the

deficiencies of traditional Traffic Engineering

(TE) techniques used to overcome the

shortcomings of IP. Multiprotocol Label Switching

(MPLS) is looked into as an ideal solution for

General and Core IP Networks. Moreover,

Multiprotocol Label Switching (MPLS) is

implemented in a network by using Open Source

Kernels and tools to check its feasibility for IP

Networks.

1. Introduction

Primarily Traffic Engineering (TE) is a term,

which refers to the techniques used to manage

traffic in a network. Now a day Traffic

Engineering is considered to be inevitable for

networks that demand more reliability and better

management of network resources. Limitations of

traditional IP Routing, discussed in section [2], can

only be dealt with different Traffic Engineering

techniques.

Figure-1 shows a simple representation of a

network scenario having TE execution. Here node

R1 can reach R2 via three different paths i.e. A, B

and C respectively. Each link has different

bandwidth and can suite different categories of

traffic. Real Time Traffic can be forced to follow a

more bandwidth rich path and same can be the

case with VOIP and E-mail services as shown in

Figure-1. In this way network resources are

utilized more efficiently and traffic receives better

service.

Well-known techniques used for Traffic

Engineering are ATM-Overlay [1] and MPLS-TE.

ATM-Overlay model has some major drawbacks,

which are discussed in section [3] of this paper.

MPLS on the other hand is considered to be the

most appropriate solution for Traffic Engineering

encompassing the features of IP and ATM.

Internet is continuously on the rise in size ever

since it came into existence, so much so that every

year it increases almost by 100% [2]. Internet in its

current state has become unwieldy and

unpredictable in terms of providing specific

service for demanding applications. These

applications can be anything from web browsers to

Peer-to-Peer (P2P) and web2 applications [3]. P2P

however, is the main application that is

contributing to rapid growth of Internet [3].

According to functionality Internet can be divided

into two main parts i.e. one as the Core Network

and another as an IP Network.

Figure-2 shows a very basic division of the

Internet into Core and IP networks. Core Network

consists of ATM Switches having Mesh topology.

This network is connection oriented in nature and

provides reliable and efficient service for traffic.

However, major concern is the IP Network, which

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lacks the performance efficiency of Core Network.

This is basically due to the complex architecture of

IP Network. Main Protocol operating in this

network is IP and it is very well suited for a

network of this kind with its Best Effort Routing.

However, Implications of Best Effort IP routing

and its incapability in Traffic Engineering is the

main concern here.

2. IP Routing Limitations

To better understand the importance of MPLS

as new TE technique we need to have an

understanding of IP Routing Limitations

Service Guarantee

Best effort routing otherwise means that there

can be no service guarantee for any kind of traffic,

which is forwarded in an IP domain. Service

Guarantee might not be an important issue for

lightweight traffic, but for time critical traffic e.g.

video and voice, service guarantee is an important

concern. Any delay or loss of data means more

jitters resulting in bad quality of traffic. IP

currently provides two methods of Quality of

Service (QoS) namely, Integrated QoS [4] and

Differentiated QoS. [5].

Class of Service (CoS) & Congestion

Awareness

IP Routing in its current state is not capable of

discriminating different types of traffic. All the

traffic in IP Routing domain is treated in the same

way regardless of the needs for network resources.

The CoS support on the basis of traffic or user is

not possible in IP, which eventually results in

under utilized or otherwise congested routes.

MPLS overcomes this problem with Forward

Equivalence Class support. Also, the Inability of

IP to find congestion or available bandwidth on a

route contributes in casual forwarding decisions.

Possible solution provided by IP to overcome this

problem is termed as Load Balancing, but due to

its simple criteria of dividing the traffic, it suffers

from scalability problem.

2.3 Scalability

IP Routing has encountered the Scalability

issue due to its routing mechanisms. In traditional

IP Routing, Control Plane (route determination)

and Forwarding Plane (traffic forwarding) are

closely integrated. Any change in either of these

two can result into overall change in the Routing

model. On the other hand routing algorithms in use

today operate slowly due to escalating size of

routing tables in routers. A report [6] issued in

2006 shows detailed analysis of routing table

sizes, which is continuously on the rise.

Improvement in IP routing model means combined

improvement in Control and Forwarding Planes

resulting in increased complexity, which does not

seem feasible in the context of routing table sizes.

At present, IP Routing execution in large networks

is inversely proportional to speed and hence is not

scalable.

2.4 IP Route Recovery

Route failures in complex Internet architecture

are common and there is an increasing need for

fast route recovery mechanisms. When a link goes

down in IP network its recovery speed depends on

three factors. First factor is the time taken by a

router to detect a link failure. Second factor is the

distribution of this information across its adjacent

router and effectively to the whole network. Third

factor is the calculation of new routing tables by

all routers and finding alternate route for traffic.

There are specific messages that Routing Protocols

use to determine whether a link is active or

inactive. However, in case of route failure routing

protocols cannot find the location of failed route.

These factors play an important role in making IP

rerouting techniques slow in convergence. Clearly

there is need for fast reroute techniques [7].

3. ATM Overlay Model & Limitations

ATM Overlay [1] model was considered to be

the superlative option for traffic engineering prior

to MPLS-TE. It is based on connection-oriented

model, which is crucial for guaranteed delivery of

data in core network. Consider Figure-2 where

data comes from IP Network into the ATM Core

Network. IP packets cannot be sent directly on

ATM routes, so the ATM Switches encapsulate IP

packets into fixed length ATM Cells. In this way

IP layer remains transparent to all ATM switches.

ATM can provide QoS with respect to different

parameters namely bandwidth, constant/variable

and unspecified bit rates. ATM arguably has the

best QoS model at present, but despite these

features Overlay model has some hitches, which

leads to the introduction of new technologies in

Core Network.

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3.1 Bandwidth Utilization

Overlay model has low bandwidth utilization

due to the overhead involved in an ATM Cell.

Moreover ATM Switch has to encapsulate the IP

packet into 53 Bytes Cell. Figure-3 depicts an

ATM cell with 5 Bytes of overhead referred to as

cell tax, which decreases the capacity of a cell to

9.5% [8]. ATM does not support variable size

Cells and IP packet must be accommodated in

remaining 48 Bytes.

3.2 IP & ATM Network Management

Poor Integration of IP and ATM signify that IP

packets need to be encapsulated in ATM cells

before routing them on core network. It means that

ATM Overlay model holds two separate networks

i.e. IP and ATM and there is a need for separate

network management schemes for each. Also

signaling protocol for both IP (OSPF etc) and

ATM (PNNI) are different and needs to function

in their own domains. Network management of a

diverse network like ATM Overlay is very obscure

and complex.

3.3 Scalability

Figure-4 is the modified version of the MPLS

execution topology used in section [6.2], and here

it is used to explain scalability issue related with

ATM Overlay Model.

Figure-7 represents the physical topology of

Overlay model whereas Figure-4 shows logical

topology of the same model. ATM Overlay forms

a mesh topology where every node has a

connection to every other node.

Increase in the number of nodes means increase

in number of adjacencies making the network

architecture complex. More adjacencies in a core

network cause another problem, which is referred

to Flooding. Flooding occurs when there is a route

failure and every node has to forward state

information to every other node. As the network

grows in size Flooding becomes a major concern

as it floods the network with route failure

information.

4. Why MPLS for TE

MPLS is seen as a hybrid solution, which

encompasses the features of IP and ATM. The

most telling feature of MPLS is its ability to

separate Control Plane and Forwarding Plane [9].

This effectively means that TE is entirely

controlled by IP without any support of layer 2

technologies, which contributes to its simplicity.

Some of the features of MPLS-TE are discussed

here. MPLS plays an important role in the

introduction of Next Generation Network (NGN)

[10].

Temporary LSP Tunnels

MPLS as a hybrid solution inherits some

features from ATM and IP. One such feature that

correlates with ATM VCs (virtual circuits) is the

tunnel creation before communication between

two parties. Unlike IP with no service guarantee

for real time data, MPLS can provide some level

of guarantee by making sure that the Tunnel is

used by one service at a time. These tunnels are

not permanent as in ATM network and are

detached after communication. Also, QoS

achieved by ATM VCs can be provided using LSP

Tunnels.

Intelligent Routing decisions

Unlike IP routing, which is based on the

shortest path and destination IP addresses,

techniques used by MPLS for routing are much

sensible. Configuration of LSR/LER can be static

or dynamic and during configuration traffic and

path constraints are taken into account. In IP

Routing there are signaling protocols (OSPF, IS-

IS), which distribute path information without

considering path and traffic constraints. In ATM a

protocol called PNNI is used for the same reason.

MPLS also needs an efficient signaling

mechanism to share label information among all

LSRs. Constraints taken into consideration are

bandwidth, latency and type of traffic. MPLS also

allows for the classification of traffic according to

service needs. This classification is helpful in

correct assignment of network resources to each

traffic class. MPLS supports different protocols for

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distributing constraint information across all

LSRs.

Scalability

IP and ATM suffer from scalability problem,

MPLS implements divide and conquer rule to

achieve scalability in case of IP routing. This

means that finding routes and routing the traffic

are distinct tasks. To send traffic across an MPLS

domain, first of all routing tables, made by routing

protocols are consulted for path information. Then

the traffic is forwarded, which does not involve

routing tables or routing protocols, but an

independent use of Shim labels. It means that

routing protocols are only limited to route

determination and increase in the network size

does not increase the load on these protocols

because labels are responsible for forwarding.

Figure-4 shows the logical topology of ATM

core network. In contrast to ATM network,

number of interconnections remains the same in

MPLS as the network grows in size. Figure-7

would be a more appropriate way of representing

physical and logical topology of MPLS domain.

MPLS addresses enhance the shortcoming of Load

Balancing in IP networks by introducing a concept

of Forward Equivalence Class (FEC), which

divides different types of traffic according to their

traffic requirements. Considering Figure-1, FEC

makes it possible to put all voice traffic into one

class e.g. A with low latency and more bandwidth.

And all mail traffic into class B with loose

network requirements. In this way MPLS can

force all voice traffic to follow route A and other

traffic to follow route B. FEC support for traffic in

MPLS represents better scalability than Load

Balancing.

Fast Reroute

MPLS implementation of route recovery is

much more proficient than IP. MPLS introduces a

concept of Label Stack, in which multiple labels

are applied in one shim label. Figure-5 is the

representation of a packet with multiple labels.

In MPLS there is a backup LSP also referred to

“Protection LSP”. Primary LSP serves as the

default route for Traffic from LER1 to LER2,

while Protection LSP acts as a redundant link for

default route in case of Route failure. When the

Primary LSP goes down all the traffic arriving at

LSR1 is examined for Label Stack. Label Stack

functions as an array with Last in First-Out (LIFO)

rule. Fast Reroute does not involve broadcasting of

failed route. This means that routers in an MPLS

domain are not entitled to recalculate new routes.

Instead new route information is carried in each

packet in the form of labels.

5. MPLS Implementation Resources

For implementation IBM P-4 workstations are

used having specialized Linux Kernel for MPLS.

Each system has two NIC cards for connectivity;

also, each system has MGEN, Netperf and

Ethereal installed for generating and analyzing

traffic patterns.

5.1 Network Design

Figure-6 shows the network design for

execution of MPLS. The topology forms an MPLS

domain with LER1 and LER2 as the core network.

Linux Fedora Core4 is installed on all nodes.

Moreover, specialized kernel for MPLS is installed

on LER1 and LER2. MPLS Kernel used here is

still under development for inclusion of advanced

features. It is an open source software project and

readily available for use and research purposes

[11].

Essential configuration for MPLS is done on

LER1 and LER2, which remains transparent to

HostA and HostB. IP addresses are assigned in

such a way that Figure-6 breaks up into three

different networks. Main idea is to send traffic

from either host across the routers to the other end

of the network. LER1 and LER2 behave as Ingress

and Egress routers depending on the direction of

traffic flow.

5.2 Experiment

Figure-7 is the network topology for this

experiment, which is basically similar to Figure-4

used for representing ATM Logical Topology.

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Figure-7a is a network with static IP Routing

and Figure-7b shows the same network with

MPLS implementation. Idea of this experiment is

to use MGEN to load this network with traffic and

discover the Interarrival time for both networks

under same traffic load.

5.3 Execution

First, the Experiment is performed on a

network with static routing, Figure-7a. MGEN

resides on R1 and R3 and is used to generate UDP

traffic. Traffic pattern used by MGEN is UDP,

which sends 5000 Bytes of data every second for

duration of 90 seconds. Traffic generated from

LER1 is in two flows, the first flow initiates as

soon as MGEN runs while the second flow starts

after 30 seconds. MGEN uses Script files to

generate traffic on the source. R3 also requires an

instance of MGEN to capture UDP traffic and

store the relevant information in a log file. Trpr

can be used to parse the log file, generated by

MGEN, in order to find Packet Loss. Table-1a and

Table-1b shows the output generated after the

experiment.

5.4 Results

Table-1a shows the statistics for Interarrival

time and Table-1b shows Packet Loss for double

flow of data for the duration of 90 seconds. This

experiment is performed by static IP route

configuration in Figure-7a.

Same experiment is repeated in a network

shown in Figure-7b but with MPLS execution. The

implementation detail of MPLS remains the same.

MGEN is executed on LER1 and the results

captured on LER2 as shown in Table-2.

This experiment is repeated several times. In

Table-2a and Table-1a there is a small but notable

difference in values, although the amount of traffic

sent and time taken is the same in both cases. This

is an interesting result in Experiment, which shows

the times of two networks loaded with same

traffic. Although this report discusses the benefits

of MPLS in terms of its speed, but the results

shown in this experiment are opposite to what

MPLS can offer. There is a small but notable

difference between the Interarrival time of traffic

in both networks and MPLS time is on the higher

side. In view of the author this is because of the

32-bit overhead involved with every packet.

MPLS operates fast because it does switching,

however, label switching in this experiment is of

no importance. The reason being, it involves extra

processing in both LERs in order to check layer3

and then attach a label only to perform switching

in one LSR. Contrastingly results achieved with

static routes in same network are better because

the routing entries are not overtly populated. And

the routes only need to look up the next hop entry

without any need of attaching an extra label. With

this observation in functionality it is safe to say

that MPLS provides switching mechanism with

the cost of overhead. Therefore, MPLS is not an

option for small networks and for networks with

no Traffic Engineering requirements.

6. Conclusion

MPLS is an emerging technology and by no

means a perfect solution to current IP network

problems. It has got advantages and disadvantages

but at the moment it provides much better Traffic

Engineering capability than ATM Overlay. MPLS

is neither a substitute for IP Routing and nor it is a

new QoS model for existing carrier networks. In

actual fact MPLS operates in coordination with IP

Routing and its main objective is to provide the

speed of switching to Layer 3. Introduction of

Labels provide an effective alternative and evades

the need of large routing table lookups and results

in fast routing. However, the telling factor of

MPLS is its ability to manage and classify the

traffic in order to provide better utilization of

resources than IP Routing. MPLS is not designed

to compete with IP or ATM but rather to provide

better integration with the network layer and link

layer technologies. On the other hand both IP and

ATM have got integration problems. ATM mainly

serves the core network and it also needs to carry

the IP traffic in ATM cells. As MPLS supports

both technologies and hence can be used to

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effectively resolve integration and traffic

engineering issues in carrier networks.

References

[1] Chuck Semeria "Traffic Engineering for the New

Public Networks",Juniper Networks, January 1999

[2] Andrew M.Odlyzko, “Internet Traffic Growth:

Source and Implications”, University of Minnesota,

Minneapolis, MN, USA, 2003

[3] http ://www.oreillynet.com/pub/a/orielly/tim/news

/2005/09/30/what-is-web-20.html

[4] B. Braden, D. Clark, and S. Shenker, "RFC1633:

Integrated Services in the Internet Architecture: an

Overview"

[5] S.Blake, D.Black, M.Carlson, E.Davies, Z.Wang,

W.Weiss, "An Architecture for Differentiated

Services"

[6] Philip Smith, "Internet Routing Table Analysis

Update", SANOG, Karachi, July 2006

[7] V.Sharma, F.Hellstrand, "Framework for Multi-

Protocol Label Switching (MPLS)-based

Recovery", February 2003

[8] Document ID:10489, "Cisco - Measuring the

Utilization of ATM PVCs"

[9] Bruce Davie, Yahok Rekhter, MPLS Technology

and Applications: Morgan Kaufmann, 2000

[10] Keith Knightson, Noataka Morita, Thomas Towle,

NGN Architecture: Generic Principles Functional

Architecture and implementation 2005

[11] http://sourceforge.net/