An Experimental Analysis on OSPF-TE Convergence Time

Transcription

1 An Experimental Analysis on OSPF-TE Convergence Time S. Huang* a, K. Kitayama a, F. Cugini b, F. Paolucci c, A. Giorgetti c, L. Valcarenghi c, P. Castoldi c a Osaka University, Osaka, Japan; b CNIT, Pisa, Italy; c Scuola Superiore Sant Anna, Pisa, Italy ABSTRACT Open shortest path first (OSPF) protocol is commonly used as an interior gateway protocol (IGP) in MPLS and generalized MPLS (GMPLS) networks to determine the topology over which label-switched paths (LSPs) can be established. Traffic-engineering extensions (network states such as link bandwidth information, available wavelengths, signal quality, etc) have been recently enabled in OSPF (henceforth, called OSPF-TE) to support shortest path first (SPF) tree calculation upon different purposes, thus possibly achieving optimal path computation and helping improve resource utilization efficiency. Adding these features into routing phase can exploit the OSPF robustness, and no additional network component is required to manage the traffic-engineering information. However, this traffic-engineering enhancement also complicates OSPF behavior. Since network states change frequently upon the dynamic trafficengineered LSP setup and release, the network is easily driven from a stable state to unstable operating regimes. In this paper, we focus on studying the OSPF-TE stability in terms of convergence time. Convergence time is referred to the time spent by the network to go back to steady states upon any network state change. An external observation method (based on black-box method) is employed to estimate the convergence time. Several experimental test-beds are developed to emulate dynamic LSP setup/release, re-routing upon single-link failure. The experimental results show that with OSPF-TE the network requires more time to converge compared to the conventional OSPF protocol without TE extension. Especially, in case of wavelength-routed optical network (WRON), introducing per wavelength availability and wavelength continuity constraint to OSPF-TE suffers severe convergence time and a large number of advertised link state advertisements (LSAs). Our study implies that long convergence time and large number of LSAs flooded in the network might cause scalability problems in OSPF-TE and impose limitations on OSPF-TE applications. New solutions to mitigate thes convergence time and to reduce the amount of state information are desired in the future. Keywords: MPLS, GMPLS, OSPF-TE, LSP, link failure, convergence time 1. INTRODUCTION Traffic engineering (TE) has been considered as an important initial application over multi-protocol label switching (MPLS) and generalized MPLS (GMPLS) [1]. TE parameters can be link bandwidth information, protection information, or especially in wavelength-routed optical network (WRON) wavelength availability and wavelength specification constraint. A major goal of TE is to facilitate efficient and reliable network operations while simultaneously optimizing network resource utilization and traffic performance. Two types of architectures have been introduced to enable TE for label-switched path (LSP). One is the architecture based upon path computation element (PCE) and the other is the architecture based upon routing protocol extension. In the PCE-based architecture, a PCE may be a network node or component. The PCE takes the responsibility for collecting TE information and performing optimal LSP computation in response to a path computation client (PCC) upon the connection request. This solution has been studied intensively by IETF working groups [2]. The motivations for PCE-based architecture are considered as its high compatibility to the existing Internet model and its capability making use of distributed centers of information or computational ability. In contrast, the routing extension-based architecture does not introduce any new separated element but instead, it just includes TE extensions in the routing protocols in routers. TE extensions have been added to routing protocols OSPF (OSPF-TE) and IS-IS (ISIS-TE) [1], and a TE LSP can be directly obtained from the shortest path first (SPF) tree. By taking advantages of the routing protocol robustness, the TE information can be easily and automatically advertised upon any network state change. The TE database in the routers needed to be synchronized to keep consistence of SPF tree in all routers. However, adding more features in the TE parameters may result in longer convergence time for the network to go back to steady states. According to our knowledge, investigation of OSPF-TE performance is still very limited. This paper aims at providing experimental insights into OSPF-TE behavior for future OSPF-TE applications. *huangshw@pn.comm.eng.osaka-u.ac.jp; phone ; fax ; Photonic Networks Laboratory 1

2 Some parameters can be considered to evaluate the OSPF-TE performance, e.g., the number of network state updating messages, routing load on processors, route flaps, convergence time (time needed to synchronize the router TED database in the network), etc. For example, A. Basu, et al. [3], studied stability issues including the convergence time by their VENUS simulator. They confirmed that the OSPF-TE protocol does converge to steady state even after link failure. A. Shaikh, et al. [4] developed a black-box method (external rather than internal instrumentations were used) to observe experimentally different internal delay components of the total convergence time by experiments using Cisco routers. Their black-box method is very useful to capture the internal behavior of OSPF. Nevertheless, these two studies do not provide an experimental investigation of the convergence time of OSPF taking into account TE. In this paper, we present a black-box measurement methodology to observe the behavior of network convergence time of OSPF-TE protocol. A monitor (Linux PC) is placed out of the emulated network to detect events causing network states to change and measure the delay until the network converges to a steady state. This paper has the following two main contributions. First, we provide the experimental convergence time measurement evaluating OSPF- TE performance upon dynamic LSP establishment and LSP re-routing under single-link failure. Second, a 32-wavelength bidirectional ring WRON introducing per wavelength specification to OSPF-TE is emulated. Our findings, based upon the experimental results, can be summarized as follows: OSPF-TE database synchronization delay is the dominant factor in OSPF-TE convergence time, OSPF-TE database with per wavelength information extension can also converge to a steady state, but suffers much longer delays than OSPF-TE with only per link bandwidth information extension, a large amount of redundant information is advertised in the case of OSPF-TE with per wavelength information extension, which increases the total network load and decreases bandwidth utilization. However, in this paper, we do not assess whether taking into account TE information in optimal path computation is good or not, but instead we prefer to show the necessity of modifying the current mechanism of OSPF-TE protocol to improve the performance in the future. It also should be noted that, this paper mentions only the intra-area network with a single domain, discussions about multi-domain network, inter-area network and inter-as (autonomous system) are out of its scope. The rest of this paper is organized as follows. In Section 2, a brief overview of OSPF-TE is presented as background knowledge. In Section 3, we describe our black-box methodology of measuring different delay components of convergence time of OSPF-TE. In Section 4, two types of experiments are described to estimate the convergence times upon dynamic LSP establishment and LSP re-routing under single-link failure based on commercial routers and some experimental results are also given. Finally, we summarize the paper briefly. 2. A BRIEF OVERVIEW OF OSPF-TE OSPF-TE is a link state routing protocol [1]. In conventional OSPF the topology information used to compute SPF tree is maintained by router LSAs and only reflects the regular link state (e.g., adjacency) regardless of the network states. But in OSPF-TE, extended link attributes are taken into account to build an extended link states database so as to create a TE topology for a given OSPF area. The extended link attributes can be bandwidth information or wavelength continuity constraint in WRON. In general, this extended TE topology does not necessarily match the regular routed network based upon router LSAs. Since the network states, such as resource availability and wavelength continuity constraints, are included in the database for SPF tree computation, optimal path computation can be achieved. In OSPF-TE, the TE extension makes use of the opaque LSA of type 1 which is with an area flooding scope. The opaque LSAs are encapsulated in a link-state update (LSU) packet. As the main payload, a TLV (type-length-value) structure is introduced to the opaque LSA to reflect the TE attributions of a point-to-point link. For an example, Fig. 1 illustrates a screen-shot of the reservable bandwidth information of link from to in the opaque LSA. This information can be advertised to other routers in the same area just as router LSAs are performed. Upon receiving the advertised opaque LSAs, routers will update their TE database and re-compute the SPF tree. In WRON without wavelength converters, since the same wavelength must be reserved along the route, using the TE database incorporating the wavelength continuity constraint in the routing phase can be beneficial to obtaining a route with such 2

3 free wavelengths. This can help to reduce the connection reject risk in the signalling phase by RSVP-TE and improve light-path establishing delay performance. Figure 1 An opaque LSA showing the reservable bandwidth information of link from to In general, the following parameters can be used to evaluate the OSPF-TE performance, network convergence time: the time taken by all OSPF-TE routers in the network to go back to steady state upon any network state change. A low convergence time indicates a stable network, number of flooded messages: packets flooded in the network. The number of flooded messages indicates the number of network changes and how many TE extensions are included in the opaque LSAs. routing load on processors: a measure of how much a router spends in processing control packets. A high routing load may imply frequent network state changes or low processing speed of the processor, routing flaps: reference to routing table changes in a router. The number of routing flaps characterizes the intensity of perturbation in the network. However, the above four parameters cannot be considered independent, e.g., high convergence time may be caused by high routing load on processors. To obtain a true optimal path from the SPF tree, TE database at all routers must be always kept consistent constantly or we say fast TE database synchronization is required to guarantee the SPF tree computation accuracy. Network convergence time is considered as a key parameter to evaluate this accuracy. Therefore, understanding the convergence time behavior and performances upon different network state changes becomes essential if TE extensions are considered in the routing phase. 3. EXPERIMENTAL METHODOLOGY In this section, we describe the methodology for investigating the OSPF-TE convergence time behaviors upon dynamic LSP establishment and LSP re-routing under single-link failure. All the experimental setups consist of an emulated network (bus or ring, etc.) and a monitor probing messages and measuring delay. Figure 2 illustrates the test-bed setup. Physical routers (Juniper networks) in which OSPF-TE implementations are available are used to construct emulated networks. The emulated network is composed of logical routers created inside the physical routers. To understand the behaviors of OSPF-TE and evaluate its performance, we developed a monitor (Linux PC). The monitor and the physical routers are connected via an Ethernet switch. In our experiments, the monitor has the following functionalities. First, it communicates with the Juniper routers by using JUNOScript, which includes generation of the emulated network topology and control of the experiments. Second, 3

4 the monitor can capture all the messages generated and flooded in the network, check the message protocol, time-stamp the packets. Figure 3 shows the screen-shot of the captured packets by the monitor, in which the convergence time can be calculated based upon the time-stamps. Figure 2 Test-bed setup for measuring convergence time by using an external monitor. Figure 3 Screen-shot of packet capturing by the monitor. The method used in our experiments is a black-box measurement method, because it relies on the external observations (using the monitor) to evaluate the OSPF-TE convergence time performance in a given network (using the emulated networks). Though the black-box method cannot guarantee measurement accuracy as high as the white-box (relying on internal instrumentation), it is still beneficial to simplifying the measurements without any modifications to the commercial routers and understand the tendency of the OSPF-TE convergence time behaviors. 4

5 4. EXPERIMENTAL DESIGNS AND RESULTS In this section, we describe three test-beds emulating dynamic LSP establishment and LSP re-routing under single-link failure by using the above black-box measurement method. For each experiment, we estimate the measurement start time ts and end time te when the corresponding messages are probed by the monitor. The actual convergence time should be expressed by (ts-te)-toverhead in which toverhead is processing overhead time (packet propagation delay from the router to the monitor, packet processing delay inside the monitor, etc.) from the actual event occurrence to the time when it is detected by the monitor. However, this overhead time is excluded from our experiments and we consider that this does not have considerable impacts on the tendency of the convergence time. 4.1 Convergence time estimation of dynamic LSP establishment Figure 4 Experimental setup for measuring convergence time of dynamic LSP establishment with a bus network. The first type of experiments is designed to evaluate the convergence time of dynamic LSP establishment with bandwidth requirement. The implementation is quite simple and is accomplished by the following procedures, - through the controlling engine, the monitor communicates to the Juniper router to create an emulated network with an eight-node bus topology which is illustrated in Fig. 4, - again through the controlling engine, the monitor sends LSP establishment request to logical router R1 in the emulated network, - upon the LSP establishment, the monitor collects the necessary messages to estimate the convergence time by its measuring engine. Figure 5 shows the event sequence upon a new LSP establishment. Symbol S represents the node at which connection request with bandwidth specification arrives. In our experiments, S refers to R1. RSVP protocol is used for resource reservation. Let tresv denote the time when S receives a successful RESV message from its downstream node. The RESV message is also probed by the monitor whose time is tpro-resv. Though tpro-resv and tresv may be slightly different upon the receiving timing, we take tpro-resv for the event start time in our experiments. After S receives the RESV messages, it will generate LSU packets to announce the bandwidth change. Let tsend-lsu denote the time when S advertises the LSU packets to its adjacent nodes. These LSU packets will be also probed by the monitor at tpro-gen. There is also 5

6 Table 1 Symbols and their meanings in the experiment upon a new LSP establishment, referring to the (a) network; (b) monitor (a) (b) Time Symbol Time Symbol RESV receiving time at S t resv RESV receiving time at monitor t pro-gen LSU generating time at S t send-lsu LSU receiving time at monitor t pro-flood LSU receiving time at downstream t rev-lsu LSU generation delay t gen LSU flooding time at downstream t flood-lsu LSU flooding delay t flood Figure 5 sequence of events during the measurement of convergence time upon a new LSP establishment.. Delay (s) Generation delay Flooding delay Number of tests (a) Percent (%) Percent Percent (%) Percent of the (%) total Percent sampled (%) Percent tests (%) (%) (a) 1-link LSP R1 to R (b) 2-link LSP R1 to R3 (c) 3-link LSP R1 to R4 (d) 4-link LSP R1 to R5 (e) 5-link LSP R1 to R Convergence time (s) (b) Figure 6 Experimental setup for measuring convergence time upon the dynamic LSP establishment with 2 Mbits/s bandwidth requirement. (a) generation delay and flooding delay upon a one-hop LSP establishment; (b) convergence time upon different length LSP establishment. 6

7 some difference between t pro-gen and t send-lsu, because t pro-gen includes the overhead of packet propagation time from S to the monitor and the packet processing time inside the monitor, but t pro-gen is taken as the time when LSU packets are generated. (t pro-gen -t pro-resv ) is the LSU generation delay upon the new LSP establishment (referred to t gen ). At t rev-lsu, the adjacent node to S receives the LSU packets and floods it to its own adjacent nodes at t flood-lsu. The flooded LSU packets are probed by the monitor again at t pro-flood, and the flooding delay is (t pro-flood -t pro-gen ). When R8 receives the LSU packets, we consider that the network goes back to steady state. Therefore, the network convergence time upon a new LSP establishment can be expressed as (t e -t pro-resv ), where t e is the time when the last LSU packet is received. (Symbols used in the experiments are shown in Table 1.) Figure 6 shows the experimental results. In Fig. 6 (a), a one-hop LSP establishment is considered. From the samples obtained in the experiment, comparing to the results of OSPF without TE extensions in [4], big differences between the LSU packet generation and flooding delays are observed in OSPF-TE considering the bandwidth information. Generation delay up to 3 seconds is observed in our experiments, which is an absolute dominant factor of convergence time. Moreover, by increasing the TE links of the LSP, it suffers longer convergence time. As shown in Fig. 6 (b), convergence time around 5 seconds is needed for a 5-hop LSP from R1 to R6. More TE links involved by the LSP increasing the size of opaque LSAs and processing load of the router can be considered as the main reason. 4.2 Convergence time estimation of LSP re-routing under single-link failure: per link bandwidth information extension versus per wavelength information extension The purpose of the second type of experiments is twofold: (1) evaluating the convergence time upon LSP re-routing under single-link failure; (2) comparing the convergence time and the number of TE messages based upon per link bandwidth information (hereafter, called CASE(link)) and per wavelength information (hereafter, named CASE(wavelength)). We consider the following implementations, - through the controlling engine, the monitor communicates to the Juniper router to create emulated networks with ring topologies (see Fig. 7 and Fig. 8). In CASE(wavelength), a link with W=32 wavelengths is emulated in the data plane by using VLANs configuration (as illustrated in Fig. 8), - in both CASE (link) and CASE (wavelength), K=32 bidirectional LSP requests between R3 and R8 are sent by the monitor. The working and backup routes are configured as illustrated in Fig. 7 and Fig. 8. Upon failure occurrence, resources are released along the working path and occupied by the backup path. - single-link failure is emulated in the physical router by disabling the interfaces. Link failure is announced by using simple network manage protocol (SNMP). The monitor collects the SNMP link-failure traps and the LSU packets informing TE information changes. Figure 9 shows the event sequence upon single-link failure. Let t fail denote the time when a link failure occurs, and t snmp and t snmp the times when two end-routers generate SNMP traps to inform the link failure. With the usual SNMP configuration, the SNMP trap can only be generated within 1 second after the actual failure detection. Since each link has two ends, the number of SNMP traps is equal to 2 W, where W refers to the number of interfaces defined in the logical routers. In CASE (link), we only have W=1 interface defined in each logical router, thus two SNMP traps are received by the monitor. In CASE (wavelength), we define W=32 interfaces taken for 32 wavelengths in each logical router, therefore 64 SNMP traps are received by the monitor. Among the traps, the time t pro-snmp when the first one is received is regarded as the event start time t s of the measurement. Upon a link failure, first, new router LSAs announcing adjacency changes are advertised. Second, the resources along the working path are released and occupied by the backup path. In both CASE (link) and CASE (wavelength), since interfaces at each router are implemented OSPF-TE, new opaque LSAs announcing the network state changes are advertised. The number of new opaque LSAs in both cases is equal to 2 N, where N is the number of involved TE links. N in CASE (link) and CASE (wavelength) is 7 and 224 (7 32), respectively. The total number of new opaque LSAs is shown in Table 3. When the last router LSA and opaque LSA are received by R8, we say that the physical network topology and TE topology converge, with convergence times equal to (t pro-snmp - t pro-rtr ) and (t pro-snmp - t pro-opq ), respectively. Here, t pro-rtt and t pro-opq are the times of receiving the last router LSA and opaque LSA. From these two convergence times, the larger is taken as the total network convergence time. 7

8 Figure 7 Experimental setup for measuring convergence time of OSPF-TE with per link bandwidth information extension. Figure 8 Experimental setup for measuring convergence time of OSPF-TE with per wavelength information extension. 8

9 Table 2 Symbols and their meanings in the experiment upon LSP re-routing under single-link failure, referring to the (a) network; (b) monitor (a) (b) Time Symbol Time Symbol SNMP trap sending time at A t snmp-trap trap probing time from A t pro-gen SNMP trap sending time at B t snmp-trap trap probing time from B t pro-flood last router LSA receiving time t rev-rtr last router LSA probing time t pro-rtr last opaque LSA receiving time t rev-opq last opaque LSA probing time t pro-opq Figure 9 Sequence of measuring procedures on the monitor upon re-routing under single-link failure. Delay (s) Router-LSA Opaque-LSA LSP 16 LSPs 32 LSPs Number of tests Number Times of of failure tests (a) (b) Figure 1 Delays upon re-routing under single-link failure taking into account per link bandwidth information. (a) delay of router LSA and opaque LSA; (b) convergence time of different number of LSPs. 9

10 Per link bandwidth TE Link failure Last LSA Packets Per wavelength TE Link failure Last LSA Packets 3 6 Time (s) (a) 3 5 Time (s) (b) Figure 11 Convergence times and the number of advertised messages of LSP re-routing under single-link failure; (a) CASE (link); (b) CASE (wavelength). Table 3 Performance comparison: per link bandwidth information extension versus per wavelength information extension CASE (link) CASE (wavelength) OSPF-TE packets 1 around 34 new opaque LSAs refresh opaque LSAs 5 around 27 convergence time 6 seconds 55 seconds Figure 1 illustrates these two convergence times and the total network convergence time of CASE (link). From Fig. 1 (a), it can be observed that the physical network topology can converge in a very short time (<.5 second), but the TE topology takes much longer (around 6 seconds). We can conclude that adding bandwidth information extension to OSPF-TE increases the total network convergence time and the TE topology convergence time is the dominant delay factor. The next consideration is whether the number of bi-directional LSPs in CASE (link) has an impact on the total network convergence time. As illustrated in Fig. 1 (b), we can observe that the number of LSPs almost has no impact on the mean convergence time. The reason is that, in CASE (link) all the changes caused by the LSPs can be summarized by one opaque LSA in a short time. New opaque LSAs are not generated one by one upon the changes caused by each LSP. In Fig. 11, we show the performance of CASE (link) and CASE (wavelength) in terms of network convergence time performance and the number of advertised messages. In CASE (link) only considering the total link bandwidth information in OSPF-TE, the network can converge after around 6 seconds with very few messages generated. In CASE (wavelength) considering per wavelength information in one link in OSPF-TE, the network can only converge after around 55 seconds and a huge number of messages are generated. Table 3 shows the detailed comparison of these two cases. From those results, we can observe that more TE information considered in OSPF-TE results in more redundant refresh TE information (opaque LSA), which in turn increases the processing load at the router and delays the new TE information (opaque LSA) advertisement. 5. CONCLUSIONS In this paper, we presented a black-box measurement method to evaluate the convergence time of OSPF-TE by using commercial routers. Three test-beds emulating dynamic LSP establishment and LSP re-routing under single-link failure were designed. A monitor was developed to control the experiments and measure the convergence time. From the 1

11 experimental results, we observed that the network can go back to a steady state even if bandwidth information extension or wavelength information extension is included in OSPF-TE. Comparing to the OSPF without TE extension, it was shown that the network spends more time converging upon any network state change. The delay of the TE database synchronization is the dominant factor of the total convergence time. Furthermore, more TE links or more TE features added to OSPF-TE result in longer convergence time. In WRON, adding per wavelength information extension to OSPF- TE suffers severe long TE database synchronization delay and a huge number of advertised messages. This implies that with the current mechanisms installed in the commercial routers, these disadvantages may impose scalability limitation on OSPF-TE applications. However, OSPF-TE still has many advantages that can simplify the network architecture from the control and management perspectives, e.g., an OSPF-TE enabled network is self-managed, each node in the network can have a global view of the network, and optimal path computation can be achieved without introducing extra equipments. Therefore, new solutions that can improve the OSPF-TE convergence time performance are desired. 6. ACKNOWLEDGEMENTS Shaowei Huang would like to thank the Japan Society for the Promotion Science (JSPS) for the financial support and all the members in the Optical Network Architecture Group in Scuola Superiore Sant Anna (SSSA) for the valuable discussions and comments on this work. REFERENCES [1] [2] [3] [4] D. Katz, K. Kompella, and D. Yeung, Traffic engineering to OSPF version 2, Request for comments 363, September 23. A. Farrel, J. P. Vasseur, and J. Ash, A path computation element (PCE)-based architecture, Request for comments 4655, August 26. A. Basu, J. G. Riecke, Stability issues in OSPF routing, in Proc. ACM SIGCOMM 1, San Diego, USA, August 21. A. Shaikh, A. Greenberg, Experience in black-box OSPF measurement, in Proc. ACM SIGCOMM 1, San Diego, USA, August