exponential and unstoppable course; at the same time there has been an increasing demand of new and more sophisticated services, therefore the technology ...
Performance Evaluation of MPLS Path Restoration Schemes using OMNET++ Marcelino Minero-Muñoz, Vicente Alarcon-Aquino, Jose Galdino García-Fierro, Roberto Rosas-Romero, Jorge RodriguezAsomoza, Oleg Starostenko Department of Computing, Electronics, and Mechatronics Communication and Signal Processing Research Group Universidad de las Americas Puebla 72820 Cholula, Puebla MEXICO E-mail: {marcelino.minero, vicente.alarcon}@udlap.mx Abstract-Multi-Protocol Label Switching (MPLS) is an alternative to integrate Internet Protocol (IP) routing and switching technologies because it provides end-to-end Quality of Service (QoS), guarantees traffic engineering, and support Virtual Private Networks (VPNs). However, MPLS must use path restoration schemes to guarantee the delivery of packets through a network. Considering that MPLS is a label switching technology, it uses a method for label distribution based on signaling protocols like LDP (Label Distribution Protocol) among others. In this paper we present the simulation of some LDP messages and assess the performance of three path restoration schemes (Haskin, Makam, and Simple Dynamic) in an MPLS network using OMNET++. The simulation results show that the Simple Dynamic scheme presents a reduced arrival time when sending a message from the source to a destination, when compared to those times obtained to Haskin and Makam schemes.
I. INTRODUCTION During the last years, the Internet growth has taken an exponential and unstoppable course; at the same time there has been an increasing demand of new and more sophisticated services, therefore the technology has had to undergo fundamental changes with respect to the usual practices developed in the mid 90s. In this super-growth environment, the Internet Service Providers (ISPs) must find a way to adjust the dramatic growth of network traffic and number of users. Furthermore, an aspect that feeds this accelerated growth in the demand is the “best effort” nature of the Internet, in which access and distribution of contents services are emphasized instead of data transport services. But there is a growing demand for services that require a higher level of capability, especially higher predictability from the Internet (a more deterministic and less random answer). Service Level Agreements (SLAs) written to meet Layer 2, Layer 3, and even Layer 4 parameters are being request by customers to fulfill this bandwidth demand [1]. To avoid having equipment specifically designed for the new Internet applications, ISPs had to adapt any
commercially available equipment. The best option seemed to be to increase the performance of the traditional routers. As infrastructure, the ATM switching equipment was the only technology that provided the required bandwidth, the packet forwarding capacities, and traffic engineering. The idea was to combine, in many ways, the effectiveness and yield of the ATM switches with the control capabilities of IP routers. The answer was the deployment of the “IP over ATM” Model (IP/ATM). The solution that IP/ATM introduces for meeting the cell tax problem is the increasing of interconnection IP nodes. This solution creates as well the problem of exponential growth n x (n 1) of the number of nodes that form the network; this slows down the process of packet forwarding made by the corresponding protocols, and so the necessity for a new technology that meets these demands. In this paper we explain and analyze the concepts of MPLS as well as performing a simulation and comparison of path restoration schemes using OMNET++. The remainder of this paper is organized as follows. Section II presents a description and operation of the MPLS protocol. In Section III, a brief explanation of the signaling protocol LDP is presented. In Section IV different path restoration schemes are introduced. Section V presents an overview of OMNET++ and the modeling of the MPLS module. Simulation results for different path restoration schemes under different scenarios are reported in Section VI. In Section VII the conclusions of this paper are presented. II. MPLS COMPONENTS AND OPERATION This section gives an overview of the terms associated with the MPLS technology. The main components associated to an MPLS network are shown in Figure 1.The function of ingress Label Edge Router (LER) is to put a label in the IP packet and forward it to the next hop in the MPLS network. This label is assigned according to the forwarding equivalence class (FEC) of the packet. In this case the IP packet is encapsulated in an MPLS Protocol Data Unit (PDU), with an MPLS shim header included in
T. Sobh et al. (eds.), Novel Algorithms and Techniques in Telecommunications and Networking, DOI 10.1007/978-90-481-3662-9_74, © Springer Science+Business Media B.V. 2010
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the packet. The main objectives of MPLS are accomplished using fixed-length labels. Figure 2.Structure of an MPLS shim header
Figure 1: MPLS domain with LSRs, LERs, two LSPs and associated FECs
These labels included in an MPLS header (shim header) are assigned considering FECs that determine the route of a datagram. The FECs are a representation of a group of packets that share the same requirements to their transport. These FECs can be used to support QoS operations (e. g. real time applications) [2]. This FEC to label relationship determine the Label Switched Path (LSP) of a datagram, from the ingress point to the egress point of the MPLS network. In the MPLS domain of Figure 1 the router R0 (an LSR), using a signaling protocol, determines that it can reach the network 172.161.0.0 through the interface S0 using the label 200. Additionally, R0 determine that using the interface S1 it can send packets to the network 140.148.0.0 using the label 400. In other words, two FECs have been established. Figure 1 also shows the LSPs associated with a specific FEC. The complete path through an MPLS network is known as LSP. The LSP or “tunnel” at both ends of the MPLS network is a concatenation of the LSP segments between each node. In this tunnel the ingress node define the type of traffic and assigns a label. According to this label, the traffic is forwarded through the LSP without further examination. At the end of the tunnel, the egress node removes the label and forwards the traffic to an external network (e. g. an IP network). This type of tunnels allows the implementation of Traffic Engineering (TE). The MPLS label, among other fields, is part of a shim header with the structure shown in Figure 2. • The Time to Live field (TTL) indicates the period of time in which the datagram is valid. • The Stack field (S) indicates the existence of additional labels assigned to the datagram. • The Experimental field (EXP) does not have a formal definition in the MPLS technology, but it is used in Cisco label switching for Class of Service (CoS). • The Label field contains a 20-bit value at the front of the packet.
Note that additional information can be associated with a label—such as CoS values—that can be used to prioritize packet forwarding. The most used terms to describe the routing tables in the MPLS technology are the Label Information Base (LIB) and Label Forwarding Information Base (LFIB). The LIB contains the labels associated to a determined address and the address itself associated with these labels. These associations are those generated in this LSR and also those received from the LSRs in the neighborhood. The LFIB table contains only the necessary information to forward a datagram to the next hop in the LSP. This information consists on local labels (to be used between two LSRs on the same LSP and created by the LSR with this LFIB) and the output labels. This table also contains information of the interface to be used to forward the traffic to the next hop. An egress LER removes the label of the IP packet and forwards it to a traditional IP network. The Label Switch Routers (LSRs) are devices capable of forwarding packets inside an MPLS network. These routers are located inside the MPLS network and are intermediate hops between the ingress and egress LERs. Their function is to examine the labels of the received packets and replace them with another label according to the routing table of the intermediate routers. III. SIGNALING PROTOCOL LDP There are several methods to make the label distribution in an MPLS network. The most used is LDP (Label Distribution Protocol). The LDP is a complex protocol and contains more procedures, characteristics, and messages than RSVP (Resource Reservation Protocol). In this section a brief description of this protocol is presented. Among all the messages defined in the RFC of LDP, the most important are the following (see Figure 3) [2]: • • • •
Hello Messages Initialization Messages Label Request Messages and Label Mapping Messages
These messages allow two LSRs to build the routing tables in each LSR with the following procedure. The LSR A sends Hello Messages (UDP messages) through all their interfaces to find out those LSRs with a direct connection to LSR A. After this, if LSR A needs to find those labels associated with an LSR B connected to one of their interfaces, say I2, it sends Initialization Messages through I2 and LSR B responds with another Initialization Message. After the exchange of these messages a session is
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established between LSR A and LSR B. Finally, Label Request and Label Mapping Messages allow communicating the FECs and labels associations form LSR A to LSR B or vice versa. A detailed description of these LDP messages can be found in [2] and [3].
Figure 4.Simple Dynamic scheme
Figure 5.Haskin scheme
Figure 3.Signaling procedure between LSRs
IV. PATH RESTORATION SCHEMES The implementation of MPLS must include a solution to a path or route failure, and thus the inclusion of path restoration schemes in this implementation is necessary. These schemes are based on the kind of failure and each one has characteristics that make it preferable over others [4], [5]. There are several path restoration schemes that are used for comparison purposes when a new architecture is proposed. Some of these schemes are Haskin [6], Makam [7] and Simple Dynamic [8]. Additionally to these schemes, there are others like Fast Rerouting, Reliable Fast Rerouting and Optimal Guaranteed Alternate Route (see e. g. [9]). The application of these schemes depends on the specific requirements of the network [5]. These schemes forward traffic around a failure in a primary route and their objective is to minimize the time of establishment of the alternate route and avoid the excessive lost of information. These schemes can be classified according to the following criteria: • Local Repair: Minimize the required time for the fail propagation. Hence, if the restoration can be realized in local manner, it can be accomplished faster. • Global Repair. Considers that the nodes and links along the primary route are protected by one restoration route. In case of failure, the restoration scheme sends a Failure Indication Signal (FIS) to the ingress node and when it receives this FIS the alternate route is activated from this node. A. Simple Dynamic Scheme This scheme uses local repair and dynamic activation. Hence the alternate route is established when the point of failure is detected (see Figure 4).
When a failure in the primary route occurs, this scheme finds an alternate route that continues from the node that detects the failure. This scheme can consider link failures as well. B. Haskin Scheme This restoration scheme uses alternate routes previously established with local repair (see Figure 5). One of the requirements of this method is that the network topology allows the establishment of the alternate route between the ingress and egress LSRs of the LSP tunnel, in such way that the alternate LSP does not share any resource with the route to be protected [6]. The main idea of this scheme is to return the traffic from the point of failure on the protected LSP to the ingress LSR, in such way that the traffic could be redirected through an alternate route between the ingress LSR and the egress LSR of the protected tunnel. The alternate route is established as follows [6]: • The initial segment of the alternate LSP is between the last hop LSR before the point of failure and the ingress LSR in opposite direction of the protected LSP. • The final segment of the alternate route is defined between the ingress LSR and the egress LSR. C. Makam Scheme This scheme uses global repair and allows dynamic and pre-negotiated activation of the alternate route (see Figure 6). However, the dynamically-established alternate routes add more time to the restoration operation compared with the pre-negotiated activation.
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Figure 6.Makam scheme
The establishment of the alternate route for this scheme is as follows [7], [9]: • When a failure is detected, the node detecting the failure sends a failure indication signal (FIS) to the ingress node. • All the packets in transit between the failure detection and the moment in which the FIS arrive to the ingress node are lost. • When the ingress node receives the FIS redirects the traffic through an alternate route to the egress node. The main difference between this scheme and the Haskin scheme is that it does not redirects the traffic from the point of failure; instead it redirects traffic form the ingress node. V. MODELLING OF AN MPLS NETWORK The simulation of the path restoration schemes and the signaling protocols is accomplished using OMNET++. This section presents the principal characteristics of this software tool. V.1. OMNET OVERVIEW OMNET++ is free object-oriented software used to simulate in modular form discrete events establishing hierarchical structures between each component. In addition OMNET++ has a Graphical User Interface (GUI) used to develop simulations of different communication networks. The communication between different modules is accomplished using different messages. The complexity of these messages is dependent of the task related to them. For example, one message can send data directly to one module or send data using connections. The parameters of each module can be used to define its behavior or to define the network topology. OMNET++ simulations can be carried out in parallel form providing results similar to those obtained in real networks [10]. OMNET++ uses a Network Editor (NED) language to the graphical construction of the network to be used in the Graphical NED (GNED) ambient. This language defines several types of modules, the parameters associated and the interconnection between them. The GNED ambient shows the result of the code generated on NED. Some simple modules are considered the active components of the network, and they are the only modules programmed in
C++ using the library omnetpp.h included in OMNET++. This language is used to program MPLS, signaling protocols LDP, and CR-LDP and the Haskin scheme. The environment for simulations Tkenv shows the results of the NED and C++ programming and allows a complete control of the simulation programmed. This ambient show a timeline for the messages created and as a consequence shows the occurrence of events on the simulation. All these variables values (scalar or vector) modified by a module event on a simulation are stored and plotted when the simulation ends. The changes in vector variables are plotted using the graphic tool Plove and the change on scalar variables are plotted using the tool Scalars [10]. V.2. MPLS MODELLING In this section we present the MPLS network modelling using the GNED ambient of OMNET++. Afterwards, the programming of the principal functions of the signaling protocols and the path restoration schemes using C++ are presented. A. MPLS Network Modelling using GNED of OMNET++ The proposed MPLS network used to implement the path restoration schemes is designed using the ambient GNED of OMNET++ and the NED language. This proposed network consists of 4 hosts, each one connected to an IPv4 router. These IPv4 routers are connected to an MPLS network that consist of 10 LSRs and 4 LER (see Figure 7). The hosts consist of 4 layers (Application, Transport, Network and Data Link) and the routers consist of 2 layers (Network and Data Link). The MPLS routers consist of 2 layers but in addition these routers show the modules of LDP and MPLS. The design of each one of these elements is shown in the following sections. B. LDP Module In this module the signaling mechanism that establishes the LSP is carried out using the LDP protocol before label distribution. This signaling mechanism consists of the following four stages: • • • •
Discovery Session establishment Label distribution LSP creation
C. MPLS Module This module assign, swap or remove labels on the messages. The first action is to assign a label to those messages that ingress to the MPLS network. This procedure is carried out by discrimination between LDP messages and an IP datagram. These messages with a TCP or UDP port number of 646 are considered LDP messages and a message with a different port number is considered to be an IP datagram.
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Figure 7.MPLS network designed on GNED.
The swap or remove label actions are carried out using the Incoming Label Map Table (ILM) to find the incoming label and then using the Next-Hop Label Forwarding Entry Table (NHLFE) to find the action to perform with this label: swap or remove. The implementation of the path restoration schemes is carried out in the MPLS and ERIB (Explicit Route Information Base) modules as follows. The link failures are declared after the source module sends the seventh datagram (the link and routers involved in a failure are declared inside the program). This link failure triggers some actions on the node that detects this failure. Concerning to the Haskin scheme, each one of the labeled frames is sent to the MPLS module where a Route Restoration Label is used to send the traffic using this restoration route. This route restoration label is used to swap the previous label of the frame and to find the output interface of the frame with this new label. This output interface is found in the ERIB. Once the relabeled frames arrive to the ingress LER, this LER carries the switchover mechanism to send this frames through the restoration path and not through the active path. VI. SIMULATION RESULTS The simulation of this MPLS network is carried out using the Tkenv tool of OMNET++. This simulation provides a file used to plot (using Scalars) the variables defined in the program and as a consequence determine the correct behavior of the path restoration schemes. The creation of the file that contains the variables to plot is programmed in C++. The scalar variables are used to plot the received, send, and lost packets. These variables are timeindependent. Figure 7 shows the active path (LER2-LSR2LSR5-LSR8-LER3) for a packet of 28 Kilobytes (sent in 19
fragments) sent form the host H2 to the host H3. A. Haskin Scheme The restoration path when a link failure is present in the proposed network is shown in Figure 7 (LER2-LSR1LSR4-LSR7-LSR10-LER3). In this network, a packet is sent form host H2 with a destination at H3. This message is fragmented and each fragment is sent through the active path until a link failure is detected. In this case the fragments are sent through the restoration path. The active path consists of the following elements: LER2-LSR2LSR5-LSR8-LER3. The LSR2 and LSR5 process the 19 fragments of the application message. The LSR5 element sent 7 fragments to LSR8 before the link failure is detected. When the link failure is detected, the remaining fragments are sent back to the LER2, using as an intermediate node the LSR2. These fragments are received at LER2 and sent through the backup path: LER2-LSR1-LSR4-LSR7LSR10-LER3. From the point of view of LER3 the first seven fragments are received from the active path and the remaining twelve fragments are received from the backup route. B. Makam Scheme The simulation of the Makam Scheme is carried out using the same network topology used for the Haskin scheme. For this scheme a packet is sent from host H2 to the host H3. This packet is divided in 19 fragments and sent through the active route until a failure occurs. If the failure occurs when the node LSR5 has processed 7 fragments, the remaining 12 are lost. The host LSR5 sent a FIS signal to LER2. When the FIS signal arrives to LER2, the host H2 retransmits a message, which is divided again in 19 fragments. This new fragments are sent through the restoration path (LER2-LSR1-LSR4-LSR7-LSR10-LER3).
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When these 19 fragments arrive to LER3 it verifies if these are duplicated fragments or new fragments, when compared with the 7 fragments previously sent through the active path. C. Simple Dynamic Scheme Using the topology shown in Figure 7, the function of the Simple Dynamic scheme is as follows. The process considers again sending packets form H2 to H3. Before a failure exists in the topology, all the fragments are sent through the active route. When the failure exists the fragments are sent through the backup path LSR5-LSR7LSR10-LER3. In the simulation carried out for this scheme, the packet sent from H2 to H3 is divided in 19 fragments and before the failure occurs, the LSR5 process 7 fragments and sent them to LSR8. When a failure occur LSR5 establish the alternate path, which is used to send to LER3 the remaining 19 fragments. In this scheme the ingress and egress LERs do not know that a failure exists inside the MPLS network. Using this scheme there are no packets lost. According to the results obtained in the simulation of path restoration schemes we can observe that the time used to send packets form the source H2 to the destination H3 using Haskin scheme is of 300 milliseconds (see Table 1). This time includes the creation of the message (170 ms) and the time used by the fragments to arrive to the destination (130 ms). The Haskin scheme is also compared to IP networks using NS-network simulator (see e.g., [11]). For the Makam scheme the results show that the sending time form H2 to H3 increase (320 ms) when compared to Haskin scheme. The simple dynamic scheme presents a reduced sending time (260 ms) when compared to Haskin or Makam schemes. This time includes a transit time from H2 to H3 of 90 ms and the time of creation of the message at H2 (170 ms). Table 1.Arrival time for simulated path restoration schemes Path Restoration Scheme
Message Time Creation at H2
Transit time from H2 to H3
Arrival Time to H3
Haskin
170 ms
130 ms
300 ms
Makam
170 ms
150 ms
320 ms
170 ms
90 ms
260 ms
Simple Dynamic
VII. CONCLUSIONS AND FUTURE WORK In this paper we have presented the simulation of three path restoration schemes for MPLS networks using OMNET++. The principal components of this simulation tool were presented and the function of each one was described. This
tool shows the great versatility to simulate MPLS networks and to obtain simulation results similar to those obtained in a real network. Furthermore, we simulate the complete functioning of the signaling protocol LDP for the distribution of labels to each component of the proposed MPLS network. Simulation results show that it could be possible to simulate more complex schemes like OGAP (Optimal and Guaranteed Alternative Path) for the path restoration in MPLS networks. This scheme among others can offer some additional characteristics to be measured like packet reordering or recovery time for link or node failures. Further work can be done to enhance the simulation of the signaling protocol or to include the simulation of the most important characteristics of CR-LDP for Traffic Engineering. Another possible improvement can be the simulation of path restoration schemes considering optical MPLS networks. REFERENCES [1] Marconi white paper, Building scalable Service Provider IP Networks, Connection-Oriented Networking Solutions, July 2000. [2] Uyless Black, MPLS and Label Switching Networks, Second Edition, New Jersey, Prentice Hall 2002. [3] Andersson L., Doolan P., Feldman N., Fredette A., Thomas B., LDP Specification, RFC3036, January 2001. [4]
V. Alarcon-Aquino, J. C. Martínez Suárez, Introduction to MPLS Networks (in Spanish). El Cid Editor, 2006.
[5] Johan Martin Olof Petersson, MPLS based recovery mechanisms, Master Thesis, University of Oslo, May 2005. [6] D. Haskin, R. Krishnan. A method for setting an alternative label switched path to handle fast reroute. Draft-haskin-mpls-fast-reroute05.txt. November 2000. [7] S. Makam, V. Sharma, K. Owens, C. Huang. Protection / Restoration of MPLS Networks. Draft-makam-mpls-protection-00.txt. October 1999. [8]
Ahn G, Chun W. “Simulator for MPLS Path Restoration and Performance Evaluation”, Joint 4th IEEE International Conference on ATM (ICATM2001) and High Speed Intelligent Internet, April 2001, pp. 32-36.
[9] Hundessa Gonfa, Lemma, Enhanced Fast Rerouting Mechanisms for Protected Traffic in MPLS Networks, Professional Thesis, Universidad Politécnica de Cataluña, 2003. [10] Varga, A., “OMNeT++, User Manual”, OMNeT ++ Discrete Event Simulation System. Available http://www.omnetpp.org/doc/manual/usman.html.
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[11] V. Alarcon-Aquino, J. C. Martinez, L. G. Guerrero-Ojeda, Y. Takahashi, MPLS/IP Analysis and Simulation for the Implementation of Path Restoration Schemes, WSEAS Transactions on Computers, Issue 6, Vol. 3, December 2004. p. 1911-1916.
