MPLS ARCHITECTURE FOR SERVICE PROVIDER

N Rajendran, K Yugandhar, Dr V P Gulati

Institute for development and research in banking technology, MasabTank,Hyderabad-500057,India

Dr S Albert Rabara

St.Joseph’s College(Autonomous),Trichirappalli-620002,India

Keywords: MPLS, Label, QoS, Servic

e Provider’s Network, Traffic Engineering, VPN

Abstract: MPLS is an emerging backbone technology for service providers, which is being deployed on a large scale

in recent days. Using MPLS, service providers can deliver different types of services like TE, QoS and IP

VPN along with adequate security according to the specific business demands, across either switched or

routed networks. This paper deals with the current problems in service providers’ network, overview of the

MPLS technology and MPLS architecture for service provider. By deploying the MPLS in the service

provider’s network, the study reveals that the throughput of the network has been improved with decreased

latency for the larger file size.

1 INTRODUCTION

MPLS (Multiprotocol Label Switching) is an

Internet Engineering Task Force (IETF) standard for

routing traffic, where labels are attached to packets

and are forwarded along the paths, which can be as

secure as private circuits. The labeled packets are

prioritized to provide end-to-end quality of service

(QoS) and performance guarantees (Rosen, E. et al.,

2001). MPLS is not just another buzzword, but

represents a technology that service providers are

buying and implementing in their backbone

networks. As service providers start to push MPLS

into their networks, network professionals will no

longer have to build large WAN infrastructures

(Hosein F. Badran, 2001).

MPLS utilizes the features of connectionless

service and c

onnection-oriented service on to a

single bridge, thereby forming a hybrid model

(Multiprotocol Label Switching (MPLS), web

Proforum Tutorials). MPLS is one of several

initiatives for enabling the delivery on the promise

of a converged network, by combining the attributes

of Layer 2 switching and Layer 3 routing into a

single entity. This technology offers the benefits of

both conventional IP forwarding and label switching

concepts. It has the underlying strength and

scalability of IP routing plus circuit switching

features such as path optimization and path

protection. This allows carriers to design and build

networks with appropriate levels of Quality of

Service (QoS) and redundancy depending on the

customer's business requirements. It also enables

carriers to build connectionless IP networks that

behave like ATM or Frame Relay networks. By

creating virtual circuit-like tunnels, service providers

can reap the benefits of ATM's QoS capabilities by

reserving bandwidth for mission-critical applications

(Victoria Fineberg, 2003).

In this paper a complete architecture of MPLS

th IP network has been discussed for service

provider’s network. Section 2 explains the current

issues of the service provider’s network. Section 3

illustrates the evolution of the MPLS and its Label

switching concept along with the terminology and

operations. The architecture of the MPLS for

service provider’s network including the format of

the MPLS Header, Protocol suite, and services like

Traffic Engineering (TE), Quality of Service (QoS),

Virtual Private Network (VPN) and MPLS security

are discussed in section 4. The experimental study

183

Rajendran N., Yugandhar K., P. Gulati V. and Albert Rabara S. (2004).

MPLS ARCHITECTURE FOR SERVICE PROVIDER.

In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 183-190

DOI: 10.5220/0001383301830190

 SciTePress

Layer 7:

Application Layer

Layer 6: Presentation Layer

Layer 5: Session Layer

Layer 4: Transport Layer

Figure 1: OSI 7 Layer ISO Model

Layer 3: Network Layer

MPLS

Layer 2: Data Link Layer

Layer 1: Physical Layer

on MPLS and the results are presented in section 5

and section 6 concludes the paper with the benefits

for service providers by deploying the MPLS in their

network.

2 PROBLEMS IN THE CURRENT

IP NETWORKS

In recent years, there is a tremendous growth in the

internet users. Due to this, there is an overhead in the

IP routing and forwarding operations (Muckai K

Girish et al., 2000) mainly in the backbone of the

Service Provider’s network.

Various issues of the conventional IP networks

are presented below:

Scalability: The size of the routing table increases

by the addition of routers, which leads to the

scalability problem. Moreover, drawbacks of Layer

2 VPN minimize the scalability factor. The

limitation of IP version 4 address space forces the

enterprises to use Network Address Translation

(NAT) and port Address Translation (PAT) to

connect their offices which is limited to some

applications and increase the latency of the

applications.

Performance: The complete packet header is

analyzed at every time in all routers. This degrades

the performance of the network.

Load: As the number of users getting added to the

network increases, there is a heavy load on the router

resulting in node failure, link failure etc.

QoS: New applications such as Voice and Video

drive the need for guaranteed bandwidth and

increased network reliability. These applications

require services, which are deterministic in nature

i.e. guaranteed service across the complete path in

network. Existing protocols do not support these

services.

3 CONCEPTS OF MPLS

MPLS (Multiprotocol Label Switching) is a

combination of two words ‘Multiprotocol’ and

‘Label switching’. Multiprotocol means that it

supports all network protocols like IPX, Apple Talk

etc. It has link Layer Independence i.e., it can work

over ATM, Frame Relay, SONET, Ethernet, Token

Ring, and FDDI etc. Label switching is the basic

operation in MPLS; the packets get forwarded based

on these labels only.

In Label switching, instead of using a destination

address to make routing decision, a number (a label)

is assigned to the packet in order to forward it to the

destination. Label switching is not a new concept, it

has been there for many years, for example tag

switching from CISCO systems, IP switching from

Ipsilon (Nokia), Aggregate Route-based IP

switching (ARIS) from IBM, Cell Switch Router

(CSR) from Toshiba and IP Navigator from Cascade

Communications.

Different vendors developed their own

proprietary label switching technology, which

provided better performance and opens wide scope

for scalability but interoperability was missing

between them. IETF came with a new solution,

which provides interoperability between these

technologies. The IETF’s MPLS working group is

responsible for standardizing a base technology for

use of label switching (Rosen, E. et al., 2001).

MPLS brings the concept of Layer 2 switching to

Layer 3. In the ISO OSI reference model, MPLS will

fit between Layer 2 and Layer 3 as shown in the

figure 1. MPLS is a hybrid model adopted by IETF

to incorporate the best properties in both packet

routing and circuit switching. As shown in figure 2

MPLS is a hybrid model through incorporating the

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IP Route

MPLS ATMSwitch

Figure 2: Hybrid odel leads to MPLS m

Forwarding

Control:

IP Router

Software

Longest-

match

Lookup

Forwardi gn

Label

Swapping

Control:

IP Router

Software

Control:

ATM

Forum

Software

Forwarding

Label

Swa

features of control plane of the IP and forwarding

plane of the ATM (Multiprotocol Label Switching

(MPLS), web Proforum Tutorials).

3.1 Terminology in MPLS

It’s a different terminology when dealing with

MPLS. Following is the list of terminology involved

in MPLS, which is specified in RFC 3031(Rosen, E.

et al., 2001).

Table 1 Terminology involved in MPLS

Label: A short fixed length physically

contiguous identifier, which is used to

identify a FEC, usually of local

significance.

FEC: Forward Equivalence Class is a group of IP

packets, which are forwarded in the same manner

(e.g., over the Same path, with the same

forwarding treatment).

LER: The entry (Ingress Router) or exit (Egress

Router) pointing to an MPLS network are known

as label edge routers (LER).

LSR: LSRs (Label Switching Router) are

responsible for swapping the labels of a packet to

ensure that packet reaches its proper destination.

LSP: Label switched path represents the complete

path through a label switched network to reach the

destination.

LDP: Label Distribution Protocol is an signaling

protocol developed for MPLS

LFIB: Label Forwarding information Base is

similar to the routing table in IP network which

maintains all the routing information of its

neighbors

3.2 Basic Operation of MPLS

All the traffic coming into the MPLS domain enters

at the ingress router (LER) and leaves at the egress

router (LER). At the Ingress router, each packet is

assigned a label and the packet is forwarded through

out the MPLS domain based on this label. At each

router, the label gets swapped with another label

called label swapping, which represents the next

router. Finally, when the packet reaches the egress

router, it discards the label from the packet and

forwards the packet based on the network layer

header.

Basic operations of MPLS are shown in figure 3

where all routers are enabled with MPLS, by

forming into MPLS domain (Hosein F. Badran,

2001). The steps involved are as follows:

S1.Routing protocols exchange routing information

to destination networks

S2.Label Distribution Protocol (LDP) establishes

label mappings to destination network.

S3.Ingress LER receives packet and assigns a label

to the packets based on FEC.

S4.LSR forwards packets using label swapping.

S5.LER at egress removes label and delivers packet

based on the network layer header.

4 ARCHITECTURE OF MPLS

MPLS architecture is the combination of Layer 2

and Layer 3 switching of the conventional IP

networks. The steps involved for a data packet to

travel through the MPLS domain (Multiprotocol

Label Switching (MPLS), web Proforum Tutorials)

are:

Label creation and distribution- Label has to be

created based on the FEC and it has to be distributed

among the routers using protocols like LDP.

MPLS ARCHITECTURE FOR SERVICE PROVIDER

185

Figure 3: Basic Operations of MPLS

Table creation at each router- LFIB has to be

fully populated at each router.

Label-switched path (LSP) creation- Label

Switched path has to be established before the

packets are forwarded.

Label insertion/table lookup- When a data

packet reaches the ingress router, based on the FEC,

‘label’ is inserted into it and indexed into the LFIB

for its next hop.

Packet forwarding- The packet gets forwarded

at every router by swapping the label.

In an MPLS domain, not all of the source traffic

is necessarily transported through the same path.

Depending on the traffic characteristics, different

LSPs could be created for packets with different CoS

(Class of Service) requirements. The main feature of

the MPLS is the separation of the control plane and

forwarding plane (Multiprotocol Label Switching

(MPLS), web Proforum Tutorials).

MPLS Control plane is responsible for

establishing the label switched path by distributing

the routing information among LSRs (Label

Switched Router) and the procedures convert this

information to LFIBs.

MPLS forwarding plane is responsible for

forwarding packets based on the values contained in

the attached labels. Forwarding plane forwards the

packets based on the label forwarding Information

Base (LFIB), which is maintained by each MPLS

node.

4.1 MPLS Header

For layer 2 technologies like Ethernet, Token Ring,

FDDI, and P-T-P Links etc, the MPLS label is

carried in Shim Header. The format of the shim

header is shown in figure 4.

Shim Header is of 32-bit fixed length, in which

Label is of 20-bits and carries the actual value of the

MPLS label, experimental field is 3-bits used to

represent the QoS to be provided, (something is

wrong here) label stack is 1-bit used at the time of

hierarchical routing and finally TTL field is 8-bits

used at the time of loop detection and prevention

(Rosen, E. et al., 2001), where TTL is an copy of IP

TTL field and it performs the same functionalities of

the IP TTL. Shim header is placed between the

Layer 3 and Layer 2 headers as shown in figure 5

when referred to the ISO reference model

(Multiprotocol Label Switching (MPLS), web

Proforum Tutorials).

4.2 Protocol Suite

The MPLS protocol suite is classified into two

categories:

• Routing protocols

• Signaling protocols

Figure 4: Format of SHIM Header

8 3 20

Label EXP S TT

The routing protocols are responsible for the

distribution of the routing information among the

LSRs

Layer2 SHIM Layer3 Layer4

Header Header Header Header

Figure5: Shim header is placed between Layer 2 header

and Layer 3 Header

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OSPF, IS-IS, BGP belongs to the routing protocols,

where as LDP, RSVP, CR-LDP and RSVP-TE will

belong to the signaling protocols The signaling

protocols are responsible for the distribution of

labels among the LSRs (Victoria Fineberg, 2003).

The label binding information is distributed by the

protocols LDP or RSVP. LDP is the proprietary

protocol developed for MPLS by IETF. It is also

possible to use the protocols OSPF, IS-IS, BGP to

distribute the label information by extending them.

4.3 Traffic Engineering

Traffic engineering (TE) deals with performance of

a network in supporting the network’s customers and

their QoS needs. MPLS is strategically significant

for Traffic Engineering because it can potentially

provide most of the functionality available from the

overlay models like IP over ATM or IP over Frame

relay, in an integrated manner, and also at a lower

cost than the currently competing alternatives (RFC

2702). The connection oriented nature of MPLS

allows SPs to implement TE in their networks and

archive a variety of goals, including bandwidth

assurance, diverse routing, load balancing, path

redundancy, and other services that lead to QoS

(Daniel, O. et al., 1999).

MPLS traffic Engineering can be possible by

two different approaches, TE-RSVP and CR-LDP

which are currently under the development by the

IETF MPLS working Group. CR-LDP is a set of

extensions to LDP specifically designed to facilitate

constraint-based routing of LSPs. Like LDP, it uses

TCP sessions between LSR peers and sends label

distribution messages along the sessions. This allows

it to assume reliable distribution of control

messages. Generic RSVP uses a message exchange

to reserve resources across a network for IP flows.

The Extensions to RSVP for LSP Tunnels enhances

generic RSVP so that it can be used to distribute

MPLS labels (George Swallow, 1999).

4.4 Quality of Service

MPLS is frequently mentioned among major Quality

of Service (QoS) technologies for packet networks.

MPLS doesn’t define a new QoS architecture; most

of the work on MPLS has focused on supporting

current IP QoS architectures (Haeryong Lee. et al.,

2000). In MPLS the QoS can be archived in two

ways

• Integrated Services (Intserv)

• Differentiated Services (DiffServ)

IntServ (Nicolas Rouhana et al., 2000) defines

per-flow QoS and uses RSVP as the signaling

mechanism used by applications to request QoS

from the network.

MPLS can support per-flow QoS

with the extensions made to RSVP to propagate

bindings between flows and labels. The way to solve

the QoS problem by integrated services alone has been

recognized as hardly scalable, due to its need to store

per-flow state at each router, and requires substantial

changes in the existing Internet architecture.

DiffServ (Le Faucheur, F et al., 2002), defines a

QoS architecture based on flow aggregates that requires

traffic to be conditioned and marked at the network

edges (Ingress node) and internal nodes to give

different QoS treatment to packets based on their

markings. MPLS packets need to carry the packet

marking in their headers because LSRs do not examine

the IP header during forwarding. A 3-bit experimental

field in the MPLS shim header as shown in figure 4 is

used for this purpose. The DiffServ functionality of an

LSR is almost identical to that provided by an IP router

with respect to the QoS treatment given to packets (per-

hop behavior in DiffServ terms) (Victoria Fineberg,

2003).

4.5 Security in MPLS using Virtual

Private Network Services

A virtual private network means a private multi-site

network created by using shared resources within a

public network. Conventional VPNs are based on

creating and maintaining a full mesh of tunnels or

permanent virtual circuits among all sites belonging

to a particular VPN, using IPSec, L2TP, L2F, GRE,

Frame Relay or ATM. MPLS based VPNs, which

are created in Layer 3, are connectionless, and

therefore substantially more scalable and easier to

build and manage than conventional VPNs (RFC

2547bis, 2001).

MPLS is becoming a more widespread

technology for providing virtual private network

(VPN) services; Components in MPLS VPN are

Customer Edge (CE) and customer (C) routers,

which placed at the customer end and Provider edge

(PE) and Provider (P) routers, which are placed at

service providers end. CE and PE routers are useful

to establish a VPN connection. Service providers’

MPLS VPN Architectural components are shown in

the figure 6.

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187

Figure 6: Service Providers MPLS VPN Architectural Components

Enterprise B

Enterprise A

Service Providers IP

Backbone

MPLS-VPN

With the deployment of MPLS VPN, both the

service providers and customers are benefited in

many ways.

MPLS VPNs provide a platform for rapid

deployment of additional value-added IP services,

including Intranets, Extranets, voice, multimedia,

and network commerce. MPLS VPNs offer

seamless integration with customer intranets and

have increased scalability over current VPN

implementations, with thousands of sites per VPN

and hundreds of thousands of VPNs per service

provider. MPLS VPNs provide IP Class of Service

(CoS), with support for multiple classes of service

within a VPN, as well as priorities amongst VPNs.

MPLS VPNs offer easy management of VPN

membership and easy provisioning of new VPNs for

rapid deployment (Jon Harrison, 2003).

MPLS architecture security is of increasing

concern to service providers and VPN customers

(Cisco systems , Security of the MPLS Architecture

). While MPLS based services are replacing the

traditional Layer 2 VPNs such as ATM or Frame

Relay, at least they should provide the same level of

security as of Layer 2 VPNs. Service Providers

offering MPLS services have specific demands for

the security of this special VPN solution.

Hiding MPLS core structure: The internal

structure of the MPLS core network i.e. provider

edge (PE) and provider (P) elements should be

invisible to outside networks. The only information

required between the customer edge (CE) and

provider edge (PE) for a routing protocol is the

address of the PE router. Except the IP address of PE

or interface of the CE, all the remaining information

like topology, addresses of provider (P) routers are

hidden from the outside world. MPLS does not

reveal unnecessary information outside, not even to

customer VPNs (Cisco systems , Security of the

MPLS Architecture )

Resistance to Attacks: Attacks in MPLS can be

possible through the routing protocols. A potential

attack could be to send an extensive number of

routes, or to flood the PE router with routing

updates. Both of these attacks could lead to Denial

of Service (DoS). To overcome this problem, ACLs

should be defined such that the routing protocol

allows traffic from the CE and not from anywhere

else. The next measure is configuring Message

Digest 5 (MD5) authentication for routing protocols.

This MD5 is available for BGP [RFC2385], OSPF

[RFC2154], and RIP2 [RFC2082]. Along with the

routing protocols, authentication configures the LDP

authentication also, and maximum number of routes

accepted per virtual routing and forwarding instance

(VRF) should be configured wherever possible

(Cisco systems , Security of the MPLS Architecture

Impossibility of Label spoofing: In MPLS, the

packets get forwarded based on the labels rather than

IP address. So the question here is whether it is

possible to spoof label (insert wrong labels into the

MPLS network from outside) like IP spoofing? It

may be possible if one knows the MPLS core

structure. As discussed earlier MPLS core is hidden

from the outside world (Ravi Sinha, 2003) ). Apart

from this for security reasons, a PE router should

never accept a packet with a label from a CE router.

So if any labeled packet comes from the CE the

packet is dropped at the PE.

IPSec: IPSec provides an additional security

over an MPLS network (Paul Brittain, 2000) ). By

using the IPSec on top of the MPLS infrastructure, it

provides

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Host A

R1 R2

Host B

Figure 7: Set up for analyzing the traffic between routers when MPLS is enabled

Ethereal -Sniffer

encryption of parts or all traffic over the MPLS core,

so that the attacker can’t sniff traffic on the core.

IPSec provides the Authentication of the end points

(probably the CE routers) and the integrity of the

traffic, which means packets can’t be changed on

their way through the core without the change being

noticed. It also provides the Replay detection, so if

IPSec Authentication Header (AH) is used, an

attacker cannot save packet flow and reply it later.

MPLS networks on their own provide a high level of

security when compared to Layer 2 VPNs. However

MPLS does not support encryption, integrity, and

authentication. By configuring the IPSec over

MPLS, the drawbacks mentioned above can be

archived (Cisco systems, Security of the MPLS

Architecture).

5 EXPERIMENTAL RESULTS

A test bed has been setup using four Cisco 3745

routers as shown in figure 7. All the routers should

be upgraded with Enterprise IOS of Cisco for

enabling MPLS. Ethereal is the sniffer used to sniff

the data between the routers. Different sizes of files

like 2MB, 4MB, 6MB, 9MB, 11MB and 23MB are

transferred from one end to other end and the traffic

between the routers is captured and analyzed using

ethereal sniffer. MPLS is enabled on links except

Host-to-Router links. The IGP protocol used is

OSPF. It is observed that the elapsed time to transfer

a file from one end to other end is less when MPLS

is enabled on the routers when compared to general

IP network.

Figure 8a: Time taken to transfer a file

Figure 8b: Average number of packets per second

The elapsed time is varying along with the size of

the file, when the file size is small the elapsed time

is almost same as the size of the file increases the

time also differs. As this set up is small not much

difference is recorded. The difference between both

the elapsed times is plotted as shown in the figure 8.

Similarly the throughput of the traffic is calculated

from average number of packets forwarded per

second. This analysis is carried out by comparing

with general IP network i.e. once without MPLS and

once with MPLS. It is observed that average number

of packets get switched are high by enabling MPLS

compared to the IP network. The results are plotted

in Figure 8a and 8b.

6 CONCLUSION

MPLS is an emerging standard rapidly gaining

acceptance by both vendors and Service Providers.

In this paper, it has been addressed the importance

issues of MPLS such as speed scalability, service

guarantee and security. The packets get processed

faster when Using MPLS because of bringing the

speed of layer 2 to layer 3. Since MPLS supports

the concept of hierarchy, it is easy to scale the

network when compared to other networks. VPN

architecture of MPLS also helps the scalability.

Packets can be assigned a priority label, making

Frame Relay and ATM like quality of service

guarantee is possible. Packets travel along tunnels in

a public network, which are a foundation for Virtual

Through put (Packets/Sec)

9.3

9.4

9.5

9.6

9.7

9.8

9.9

10 . 1

10 . 2

With M P LS With out M P LS

Elapsed time to transfer a file

500

1000

1500

2000

2500

3000

3500

4000

24691123

Size of the File

sec

Without MPLS

With M P LS

MPLS ARCHITECTURE FOR SERVICE PROVIDER

189

Private Networks (VPNs) provides the security

along with the protocols like IPSec.

This paper addressed the problems of existing

networks mainly IP networks and how service

providers can overcome them by deploying the

MPLS in their backbone. There is greater scope for

improvements in this technology and more features

could be embedded. We intend to refine the

architecture and continue our developments efforts.

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