MPLS Feasibility for General & Core IP Networks using Open-Source Systems Syed Adeel Ali Shah (
[email protected]) Laeeq Ahmed (
[email protected]) Department of Computer Science and Information Technology N-W.F.P University of Engineering and Technology, Peshawar, Pakistan. 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.
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-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.
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. IP Routing Limitations To better understand the importance of MPLS as new TE technique we need to have an understanding of IP Routing Limitations
2.4 IP Route Recovery
2.1 Service Guarantee
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].
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].
2.2 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.
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.
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
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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.
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.
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].
3.2 IP & ATM Network Management
4.1 Temporary LSP Tunnels
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.
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.
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.
4.2 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, ISIS), 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
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
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distributing constraint information across all LSRs.
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.
4.3 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.
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.
4.4 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.
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.
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
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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.
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.
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
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.
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effectively resolve integration and engineering issues in carrier networks.
traffic
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