Jaudelice C. de Oliveira Drexel University JP Vasseur Cisco Systems, Inc Leonardo C. Chen Verizon Laboratories Caterina Scoglio Georgia Institute of Technology IETF Internet Draft Expires: March, 2004 October, 2003 <draft-deoliveira-diff-te-preemption-02.txt> LSP Preemption Policies for MPLS Traffic Engineering Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are Working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract When the establishment of a higher priority LSP requires the preemption of a set of lower priority LSPs, a node has to make a local decision on the set of preemptable LSPs and select de Oliveira, Vasseur, Chen, and Scoglio 1 draft-deoliveira-diff-te-preemption-02.txt October, 2003 which LSPs will be preempted, based on a certain objective, in order to accommodate the newly signaled higher priority LSP. The preempted LSPs are then rerouted by their respective Head-end LSR. A preempted TE LSP can either be hard preempted (default mode as defined in RFC3209) or soft preempted ([SOFT-PREPT]). In the former case, the preemption results in clearing the corresponding state that provokes a traffic disruption. In the later case (soft preemption), the Head-end LSR of a soft preempted TE LSP is notified such that it can perform a non- disruptive reroute, using the so-called Make before break mechanism. This draft documents a preemption policy that can be modified in order to stress different objectives: preempt the lowest priority LSPs, preempt the minimum number of LSPs, preempt the exact required bandwidth in order to fit the new LSP (or the set of TE LSPs that provide the closest amount of bandwidth to the required bandwidth for the preempting TE LSPs in order to minimize the bandwidth wastage), preempt the LSPs that will have the maximum chance to be reroutable. Simulation results are given and a comparison among several different policies, with respect to preemption cascading, number of preempted LSPs, priority, wasted bandwidth and blocking probability is also included. 1. Motivation Work is currently ongoing in the IETF Traffic Engineering Working Group to define the requirements and protocol extensions for DiffServ-aware MPLS Traffic Engineering (DS-TE) [DSTE-REQ,DSTE-PROTO]. Several Bandwidth Constraint models for use with DS-TE have been proposed [BC-RD,BC-MAM, BC-MAR] and their performance was analyzed with respect to the use of preemption. Recently, a non-disruptive rerouting mechanism for preempted TE LSPs was proposed in [SOFT-PREPT]. Preemption can be used to assure that high priority LSPs can be always routed through relatively favorable paths. Preemption can also be used to implement various prioritized access policies as well as restoration policies following fault events [TE-REQ]. Although not a mandatory attribute in the traditional IP world, preemption becomes indeed a very important element especially in networks using on line distributed CSPF strategies for their TE LSP path computation to limit the impact of bandwidth fragmentation. Moreover, preemption is an attractive strategy in a Differentiated Services scenario [DEC-PREP,ATM-PREP]. Nevertheless, in the DS-TE approach, whose issues and requirements are discussed in [DSTE-REQ], the de Oliveira, Vasseur, Chen, and Scoglio 2 draft-deoliveira-diff-te-preemption-02.txt October, 2003 preemption policy is considered an important piece on the bandwidth reservation and management puzzle, but no preemption strategy is defined. Note that preemption also plays an important role in regular MPLS Traffic Engineering environments (with a single pool of bandwidth). This draft proposes a flexible preemption policy that can be adjusted in order to stress the desired preemption criteria: priority of LSPs to be preempted, number of LSPs to be preempted, amount of bandwidth preempted, blocking probability. The implications (cascading effect, bandwidth wastage, priority of preempted LSPs) of selecting a certain order of importance for the criteria are discussed for the examples given. 2. Introduction In [TE-REQ], issues and requirements for Traffic Engineering in an MPLS network are highlighted. In order to address both traffic oriented and resource oriented performance objectives, the authors point out the need for priority and preemption parameters as Traffic Engineering attributes of traffic trunks. The notion of preemption and preemption priority is defined in [TEWG-FW], and preemption attributes are defined in [TE-REQ]. A traffic trunk is defined as an aggregate of traffic flows belonging to the same class which are placed inside an LSP [DSTE-REQ]. In this context, preemption is the act of selecting an LSP which will be removed from a given path in order to give room to another LSP with a higher priority (lower preemption number). More specifically, the preemption attributes determine whether an LSP with a certain setup preemption priority can preempt another LSP with a lower holding preemption priority from a given path, when there is a competition for available resources. A TE LSP may then be either hard of soft preempted [SOFT-PREPT] to avoid service disruption. For readability, a number of definitions from [DSTE-REQ] are repeated here: Class-Type (CT): the set of Traffic Trunks crossing a link that is governed by a specific set of Bandwidth constraints. CT is used for the purposes of link bandwidth allocation, constraint based routing and admission control. A given Traffic Trunk belongs to the same CT on all links. de Oliveira, Vasseur, Chen, and Scoglio 3 draft-deoliveira-diff-te-preemption-02.txt October, 2003 TE-Class: A pair of: i. a Class-Type ii. a preemption priority allowed for that Class-Type. This means that an LSP transporting a Traffic Trunk from that Class-Type can use that preemption priority as the set-up priority, as the holding priority or both. By definition there may be more than one TE-Class using the same CT, as long as each TE-Class uses a different preemption priority. Also, there may be more than one TE-Class with the same preemption priority, provided that each TE-Class uses a different CT. The network administrator may define the TE-Classes in order to support preemption across CTs, to avoid preemption within a certain CT, or to avoid preemption completely, when so desired. To ensure coherent operation, the same TE-Classes must be configured in every Label Switched Router (LSR) in the DS-TE domain. As a consequence of a per-TE-Class treatment, the Interior Gateway Protocol (IGP) needs to advertise separate Traffic Engineering information for each TE-Class, which consists of the Unreserved Bandwidth (UB) information [DSTE-PROTO]. The UB information will be used by the routers, checking against the bandwidth constraint model parameters, to decide whether preemption is needed. Details on how to calculate the UB are given in [DSTE-PROTO]. 3. LSP Setup Procedure and Preemption A new LSP setup request has two important parameters: bandwidth and preemption priority. The set of LSPs to be preempted can be selected by optimizing an objective function that represents these two parameters, and the number of LSPs to be preempted. More specifically, the objective function could be any or a combination of the following [DEC-PREP, ATM-PREP]: * Preempt the LSPs that have the least priority (preemption priority). The QoS of high priority traffics would be better satisfied and the cascading effect described below can be limited. * Preempt the least number of LSPs. The number of LSPs that need to be rerouted would be lower. * Preempt the least amount of bandwidth that still satisfies the request. Resource utilization would be improved. de Oliveira, Vasseur, Chen, and Scoglio 4 draft-deoliveira-diff-te-preemption-02.txt October, 2003 * Preempt LSPs that minimize the blocking probability (risk that preempted TE LSP cannot be rerouted). After the preemption selection phase is finished, the selected LSPs must be torn down if hard preempted (and possibly rerouted), releasing the reserved bandwidth. If soft preempted, as described in [SOFT-PREPT] their head-end is notified to perform a TE reroute. If the soft- preempted is not rerouted after a timer has expired, then the TE LSP is torn down. The new LSP is established, using the currently available bandwidth. The UB information is then updated via receipt of an IGP-TE update and/or after having simply pruned the link where preemption occurred. Figure 1 shows a flowchart that summarizes how each LSP setup request is treated in a preemption-enabled scenario. LSP Setup Request (TE-Class i, bw=r) | | v NO UB[TE-Class i] >= r ? -------> Reject LSP Setup and flood an updated IGP-TE | LSA/LSP |YES v NO Preemption Needed ? -------> Setup LSP/Update UB if a threshold is | crossed | YES v Preemption ----> Setup LSP/Reroute Preempted LSPs Algorithm Update UB Fig. 1: Flowchart for LSP setup procedure. In [DEC-PREP], the authors propose connection preemption policies that optimize the discussed criteria in a given order of importance: number of LSPs, bandwidth, and priority; and bandwidth, priority, and number of LSPs. The novelty in this draft's approach is to propose an objective function that can be adjusted by the service provider in order to stress the desired criteria. No particular criteria order is enforced. Moreover, a new criterion is added to the objective function: optimize the blocking probability (the risk that an LSP will not find a new path in which it can be rerouted). de Oliveira, Vasseur, Chen, and Scoglio 5 draft-deoliveira-diff-te-preemption-02.txt October, 2003 4. Preemption Cascading The decision of preempting an LSP may cause other preemptions in the network. This is called preemption cascading effect and different cascading levels may be achieved by the preemption of a single LSP. The cascading levels are defined in the following manner: when an LSP is preempted and rerouted without causing any further preemption, the cascading is said to be of level 0. However, when a preempted LSP is rerouted and in order to be established in the new route it also causes the preemption of other LSPs, the cascading is said to be of level 1, and so on. Preemption cascading is not desirable and therefore policies that minimize it are of interest. Typically, this can result in severe network instabilities. In the following, a new versatile preemption heuristic will be presented. In the next Section, preemption simulation results will be discussed and the cascading effect will be analyzed. 5. Preemption Heuristic 5.1. Preempting Resources on a Path It is important to note that once a request for an LSP setup arrives, each LSR along the TE LSP path checks the available bandwidth on its outgoing link For the links in which the available bandwidth is not enough, the preemption policy needs to be activated in order to guarantee the end-to-end bandwidth reservation for the new LSP. This is a distributed approach, in which every node on the path is responsible to run the preemption algorithm and determine which LSPs would be preempted in order to fit the new request. A distributed approach may sometimes not lead to an optimal solution. In another approach, a manager entity runs the preemption policy and determines the best LSPs to be preempted in order to free the required bandwidth in all the links that compose the path. The preemption policy would try to select LSPs that overlap with the path being considered (preempt a single LSP that overlaps with the route versus preempt a single LSP on every link that belongs to the route). Both centralized and distributed approaches have its advantages and drawbacks. A centralized approach would be more precise, but requires that the whole network state be stored and updated accordingly, which raises scalability issues. In a network where LSPs are mostly static, an off-line decision can be made to reroute LSPs and the centralized de Oliveira, Vasseur, Chen, and Scoglio 6 draft-deoliveira-diff-te-preemption-02.txt October, 2003 approach could be appropriate. However, in a dynamic network in which LSPs are setup and torn down in a frequent manner because of new TE LSPs, bandwidth increase, reroute due to failure, etc., the correctness of the stored network state could be questionable. Moreover, the set up time is generally increased compared to a distributed solution. In this scenario, the distributed approach would bring more benefits, even when resulting in a non-optimal solution (The gain in optimality of a centralized approach compared to a distributed approach depends on many factors: network topology, traffic matrix, TE strategy, etc.). A distributed approach is also easier to be implemented due to the distributed nature of the current Internet protocols. Since the current Internet routing protocols are essentially distributed, a decentralized approach was selected for the LSP preemption policy. The parameters required by the new preemption policies are currently available for protocols such as OSPF and IS-IS. 5.2. Preemption Heuristic Algorithm Consider a request for a new LSP setup with bandwidth b and setup preemption priority p. When preemption is needed, due to lack of available resources, the preemptable LSPs will be chosen among the ones with lower holding preemption priority (higher numerical value) in order to fit r=b-Abw(l). The constant r represents the actual bandwidth that needs to be preempted (the requested, b, minus the available bandwidth on link l: Abw(l)). L is the set of active LSPs having a holding preemption priority lower (numerically higher) than p. So L is the set of candidates for preemption. b(l) is the bandwidth reserved by LSP l in L, expressed in bandwidth modules, and p(l) is the holding preemption priority of LSP l. In order to represent a cost for each preemption priority, an associated cost y(l) inversely related to the holding preemption priority p(l) is defined. For simplicity, a linear relation y(l)=8-p(l) is chosen. y is a cost vector with L components, y(l). b is as a reserved bandwidth vector with dimension L, and components b(l). Concerning the objective function, four main objectives can be reached in the selection of preempted LSPs: * minimize the priority of preempted LSPs, * minimize the number of preempted LSPs, * minimize the preempted bandwidth, * minimize the blocking probability. de Oliveira, Vasseur, Chen, and Scoglio 7 draft-deoliveira-diff-te-preemption-02.txt October, 2003 To have the widest choice on the overall objective that each service provider needs to achieve, the following equation was defined (for simplicity chosen as a weighted sum of the above mentioned criteria): H(l)= alpha y(l) + beta 1/b(l) + gamma (b(l)-r)^2 + theta b(l) In this equation, alpha y(l) represents the cost of preempting LSP l, beta 1/b(l) represents the choice of a minimum number of LSPs to be preempted in order to fit the request r, gamma (b(l)-r)^2 penalizes a choice of an LSP to be preempted that would result in high bandwidth wastage, and theta b(l) represents the choice of preempting small LSPs, with higher rerouting probability. Coefficients alpha, beta, gamma, and theta are suitable weights that can be configured in order to stress the importance of each component in H. The coefficient theta is defined such that theta = 0 if gamma > 0. The reason for that is that when trying to minimize the blocking probability of preempted LSPs, the heuristic gives preference to preempting several small LSPs (therefore gamma, which is the weight for minimizing the preempted bandwidth enforcing the selection of LSPs with similar amount of bandwidth as the requested, needs to be set as zero). The selection of several small LSPs in a normally loaded portion of the network will increase the chance that such LSPs are successfully rerouted. Moreover, the selection of several small LSPs may not imply preempting much more than the required bandwidth (resulting in low bandwidth wastage), as it will be seen in the discussed examples. When preemption is to happen in a heavy loaded portion of the network, to minimize blocking probability, the heuristic will select fewer LSPs for preemption in order to increase the chance of rerouting. H is calculated for each LSP in L. The LSPs to be preempted are chosen as the ones with smaller H that add enough bandwidth to accommodate r. When sorting LSPs by H, LSPs with the same value for H are ordered by bandwidth b, in increasing order. For each LSP with repeated H, the algorithm checks whether the bandwidth b assigned to that LSP only is enough to satisfy r. If there is no such LSP, it checks whether the bandwidth of each of those LSPs, added to the previously preempted LSPs' bandwidth is enough to satisfy r. If that is not true for any LSP in that repeated H value sequence, the algorithm preempts the LSP that has the larger amount of bandwidth in the sequence, and keeps preempting in decreasing order of b until r is satisfied or the sequence is finished. If the sequence is finished and r is not satisfied, the algorithm again selects LSPs to be preempted based on an increasing order of H. More details on the algorithm are given in [PREEMPTION]. de Oliveira, Vasseur, Chen, and Scoglio 8 draft-deoliveira-diff-te-preemption-02.txt October, 2003 When the objective is to minimize blocking, the heuristic will follow two options on how to calculate H: * If the link in which preemption is to happen is normally loaded, several small LSPs will be selected for preemption using H(l)= alpha y(l) + theta b(l). * If the link is overloaded, few LSPs are selected using H(l)= alpha y(l) + beta 1/b(l). 6. Examples 6.1. Simple Case: Single Link We first consider a very simple case, in which the path considered for preemption is composed by a single hop. The objective of this example is to illustrate how the heuristic works. On the next section we will study a more complex case in which the preemption policies are being tested on a network. Consider a link with 16 LSPs with reserved bandwidth b in Mbps, preemption holding priority p, and cost y, as shown in Table 1. In this example, 8 TE-Classes are active. The preemption here is being performed on a single link as an illustrative example. ------------------------------------------------------------------ LSP L1 L2 L3 L4 L5 L6 L7 L8 ------------------------------------------------------------------ Bandwidth (b) 20 10 60 25 20 1 75 45 Priority (p) 1 2 3 4 5 6 7 5 Cost (y) 7 6 5 4 3 2 1 3 ------------------------------------------------------------------ LSP L9 L10 L11 L12 L13 L14 L15 L16 ------------------------------------------------------------------ Bandwidth (b) 100 5 40 85 50 20 70 25 Priority (p) 3 6 4 5 2 3 4 7 Cost (y) 5 2 4 3 6 5 4 1 ------------------------------------------------------------------ Table 1: LSPs in the considered link. A request for an LSP establishment arrives with r=175 Mbps and p=0 (highest possible priority, which implies that all LSPs with p>0 in Table 1 will be considered when running the algorithm). Assume Abw(l)=0. de Oliveira, Vasseur, Chen, and Scoglio 9 draft-deoliveira-diff-te-preemption-02.txt October, 2003 If priority is the only important criterion, the network operator configures alpha=1, beta=gamma=theta=0. In this case, LSPs L6, L7, L10, L12 and L16 are selected for preemption, freeing 191 bandwidth units to establish the high priority LSP. Note that 5 LSPs were preempted, but all with priority level between 5 and 7. In a network in which rerouting is an expensive task to perform (and the number of rerouted TE LSPs should be as small as possible), one might prefer to set beta=1 and alpha=gamma=theta=0. LSPs L9 and L12 would then be selected for preemption, adding up to 185 bandwidth units (less wastage than the previous case). The priorities of the selected LSPs are 3 and 5 (which means that they might themselves preempt some other LSPs when rerouted). Suppose the network operator decides that it is more appropriate to configure alpha=1, beta=10, gamma=0, theta=0 (the parameters were set to values that would balance the weight of each component, namely priority and number, in the cost function), because in this network rerouting is very expensive, LSP priority is important, but bandwidth is not a critical issue. In this case, LSPs L7, L12 and L16 are selected for preemption. This configuration resulted in a smaller number of preempted LSPs when compared to the first case, and the priority levels were kept between 5 and 7. To take into account the number of LSPs preempted, the preemption priority, and the amount of bandwidth preempted, the network operator may set alpha > 0, beta > 0, and gamma > 0. To achieve a balance among the three components, the parameters need to be normalized. Aiming for a balance, the parameters could be set as alpha=1, beta=10 (bringing the term 1/b(l) closer to the other parameters), and gamma=0.001 (bringing the value of the term (b(l)-r)^2 closer to the other parameters). LSPs L7 and L9 are selected for preemption, resulting in exactly 175 bandwidth units and with priorities 3 and 7 (note that less LSP are preempted but they have a higher priority which may result in a cascading effect). If the minimization of the blocking probability is the criteria of most interest, the cost function could be configured with theta=1, alpha=beta =gamma=0. In that case, several small LSPs are selected for preemption: LSPs L2, L4, L5, L6, L7, L10, L14, and L16. Their preemption will free 181 Mbps in this link, and because the selected LSPs have small bandwidth requirement there is a good chance that each of them will find a new route in the network. From the above example, it can be observed that when the priority was the highest concern and the number of preempted LSPs was not an issue, 5 LSPs with the lowest priority were selected for preemption. When only de Oliveira, Vasseur, Chen, and Scoglio 10 draft-deoliveira-diff-te-preemption-02.txt October, 2003 the number of LSPs was an issue, the minimum number of LSPs was selected for preemption: 2, but the priority was higher than in the previous case. When priority and number were important factors and a possible waste of bandwidth was not an issue, 3 LSPs were selected, adding more bandwidth than requested, but still with low preemption priority. When considering all the parameters but the blocking probability, the smallest set of LSP was selected, 2, adding just enough bandwidth, 175 Mbps, and with priority levels 3 and 7. When the blocking probability was the criteria of interest, several (8) small LSPs were preempted. The bandwidth wastage is low, but the number of rerouting events will increase. Given the bandwidth requirement of the preempted LSPs, it is expected that the chances of finding a new route for each LSP will be high. 6.2. Dynamic Case For these experiments, we consider a 150 nodes topology with an average network connectivity of 3. 10% of the nodes in the topology have a degree of connectivity of 6. 10% of the links are OC3, 70% are OC48, and 20% are OC192. Two classes of TE LSPs are in use: Voice/AToM LSPs and Data (Internet/VPN) LSPs. For each class of TE LSP, the set of preemptions (and the proportion of LSPs for each preemption) and the size distributions are as follows (a total of T LSPs is considered): T: total number of TE LSPs in the network (T = 18,306 LSPs) Voice/AToM: Number: 20% of T Preemption: 0, 1 and 2 Size: uniform distribution between 30M and 50M Internet/VPN TE: Number: 4% of T Preemption 3 Size: uniform distribution between 20M and 50M Number: 8% of T Preemption 4 Size: uniform distribution between 15M and 40M Number: 8% of T Preemption 5 Size: uniform distribution between 10M and 20M de Oliveira, Vasseur, Chen, and Scoglio 11 draft-deoliveira-diff-te-preemption-02.txt October, 2003 Number: 20% of T Preemption 6 Size: uniform distribution between 1M and 20M Number: 40% of T Preemption 7 Size: uniform distribution between 1K and 1M - LSP set up distribution LSPs are set up mainly due to network failure: a link or a node failed and LSPs are rerouted. The network failure events were simulated with two functions: - Constant: 1 failure chosen randomly among the set of links every 1 h. - Poisson process with interarrival average=1h. Table 2 shows the results for simulations with constant failure. The simulations were run with the preemption heuristic configured to balance different criteria (left side of the table) and also with different preemption policies that consider the criteria in a given order of importance rather than balancing the same (right side of the table). The proposed heuristic was configured to balance the following criteria: HPB : The heuristic with priority and bandwidth wastage as the most important criteria (alpha=10, beta=0, gamma=0.001, theta=0) HBlock : The heuristic considering the minimization of blocking probability (normal loaded links: alpha=1, beta=0, gamma=0, theta=0.01) (heavily loaded links: alpha=1, beta=10) HNB : The heuristic with number of preemptions and wasted bandwidth in consideration (alpha=0, beta=10, gamma=0.001, theta=0) Other algorithms that consider the criteria in a given order of importance: P : Sorts candidate LSPs by priority only. PN : Sorts the LSPs by priority, and for cases in which the priority is the same, orders those LSPs by decreasing bandwidth (selects larger LSPs for preemption in order to minimize number of preempted LSPs). de Oliveira, Vasseur, Chen, and Scoglio 12 draft-deoliveira-diff-te-preemption-02.txt October, 2003 PB : Sorts the LSPs by priority, and for LSPs with the same priority, sort those by crescent bandwidth (select smaller LSPs in order to reduce bandwidth wastage) ------------------------------------------------- | Heuristic | Other algorithms | ------------------------------------------------- | HPB | HBlock| HNB | P | PN | PB | ----------------------------------------------------------------- Need to be | 532 | 532 | 532 | 532 | 532 | 532 | Rerouted | | | | | | | ----------------------------------------------------------------- Preempted | 612 | 483 | 619 | 504 | 477 | 598 | ----------------------------------------------------------------- Rerouted |467|76%|341|70%|475|77%|347|69%|335|70%|436|73%| Blocked |145|24%|142|30%|144|23%|157|31%|142|30%|162|27%| ----------------------------------------------------------------- Max Cascading | 4.5 | 3(12) | 5 | 2.75 | 2 | 2.75 | Wasted Bandwidth|------------------------------------------------ AVR (Mbps) | 6638 | 532 | 6479 | 8247 | 8955 | 6832 | Worst Case(Mbps)| 35321 |26010 |36809 | 28501 | 31406 | 23449 | Priority |------------------------------------------------ Average | 6 | 6.5 | 5.8 | 6.6 | 6.6 | 6.6 | Worst Case | 1.5 | 3.8 | 1.2 | 3.8 | 3.8 | 3.8 | Extra Hops |------------------------------------------------ Average | 0.23 | 0.25 | 0.22 | 0.25 | 0.25 | 0.23 | Worst Case | 3.25 | 3 | 3.25 | 3 | 3 | 2.75 | ----------------------------------------------------------------- Table 2: Simulation results for constant network failure: 1 random failure every hour. From Table 2, we can conclude that among the heuristic (HPB, HBlock, HNB) results, HBlock resulted in the smaller number of LSPs being preempted. More importantly, it also resulted in the overall smaller rejection rate and smaller average wasted bandwidth (and second overall smaller worst case wasted bandwidth.) Although HBlock does not try to minimize the number of preempted LSPs, it ends up doing so, because it preempts LSPs with lower priority mostly, and therefore it does not propagate cascading much further. Cascading effect was only one level higher than the overall lowest cascading effect (and only 12 cases of cascading level 3 occurred). The average and worst preemption priority was very satisfactory (preempting mostly lowest priority LSPs, like the other algorithms P, PN, and PB). de Oliveira, Vasseur, Chen, and Scoglio 13 draft-deoliveira-diff-te-preemption-02.txt October, 2003 When HPB was in use, more LSPs were preempted as a consequence of the higher cascading effect. That is due to the heuristicÝs choice of preempting LSPs that are very similar in bandwidth size to the bandwidth size of the preemptor LSP (which can result in preempting a higher priority LSP and therefore causing cascading). The wasted bandwidth was improved when compared to the other algorithms (P, PN, PB). When HNB was used, cascading was higher than the other cases, due to the fact that LSPs with higher priority could be preempted. When compared to P, PN or PB, the heuristic HNB preempted more LSPs (In fact, it preempted the larger number of LSPs overall, which clearly shows the cascading effect), but the average wasted bandwidth was smaller, although not as small as HBlockÝs (the HNB heuristic tries to preempt a single LSP, meaning it will preempt LSPs that have a reserved bandwidth similar to the actual bandwidth needed. The algorithm is not always successful, because such a match may not exist and it that case, the wasted bandwidth could be high). The preempted priority was the highest on average and worse case, which also shows why the cascading level was also the highest (the heuristic tries to select LSPs for preemption without looking at their priority levels). In summary, this policy resulted in a poor performance. Policy PN resulted in the small number of preempted LSPs overall and small number of not successfully rerouted LSPs. Cascading is low, but bandwidth wastage is very high (overall highest bandwidth wastage). Moreover, in several cases in which rerouting happened on portions of the network that were underloaded, the heuristic HBlock preempted a smaller number of LSPs than PN. Policy P selects a larger number of LSPs (when compared to PN) with low priority for preemption, and therefore it is able to successfully reroute less LSPs when compared to HBlock, HPB, HNB, or PN. The bandwidth wastage is also higher when compared to any of the heuristic results or to PB, and it could be worse if the network had LSPs with low priority and large bandwidth, which is not the case. Policy PB, when compared to PN resulted in larger number of preempted LSPs and overall larger number of LSPs destroyed, due to preemption. Cascading was slightly higher. Since the selected LSPs have low priority, they are not able to preempt much further and are blocked when the links are congested. Bandwidth wastage was smaller since the policy tries to minimize wastage, and preempted priority was about the same on average and worst case. de Oliveira, Vasseur, Chen, and Scoglio 14 draft-deoliveira-diff-te-preemption-02.txt October, 2003 The simulation results show that when preemption is based on priority, cascading is not critical since the preempted LSPs will not be able to propagate preemption much further. When the number of LSPs is considered, fewer LSPs are preempted and the chances of rerouting increases. When bandwidth wastage is considered, smaller LSPs are preempted in each link and the wasted bandwidth is low. The heuristic seems to combine these features, yielding the best results, especially in the case of blocking probability. When the heuristic was configured to minimize blocking probability (HBlock), small LSPs with low priority were selected for preemption on normally loaded links and fewer (larger) LSPs with low priority were selected on congested links. Due to their low priority, cascading was not an issue. Several LSPs were selected for preemption, but the rate of LSPs that were not successfully rerouted was the lowest. Since the LSPs are small, it is easier to find a new route in the network. When selecting LSPs on a congested link, fewer larger LSPs are selected improving load balance. Moreover, the bandwidth wastage was the overall lowest. In summary, the heuristic is very flexible and can be configured according to the network providerÝs best interest regarding the considered criteria. For several cases, the failure of a link resulted in no preemption at all (all LSPs were able to find an alternate path in the network) or resulted in preemption of very few LSPs and subsequent successfully rerouting of the same with no cascading effect. It is also important to note that for all policies in use, the number of extra hops when LSPs are rerouted was not critical, showing that preempted LSPs can be rerouted on a path with the same length or a path that is slightly longer in number of hops. 9. Security Considerations The practice described in this draft does not raise specific security issues beyond those of existing TE. 10. Acknowledgment We would like to acknowledge input and helpful comments from Francois Le Faucheur (Cisco Systems, Inc.) and George Uhl (Swales Aerospace). de Oliveira, Vasseur, Chen, and Scoglio 15 draft-deoliveira-diff-te-preemption-02.txt October, 2003 References [DSTE-REQ] F. Le Faucheur and W. Lai, "Requirements for support of Differentiated Services-aware MPLS Traffic Engineering," RFC 3564, July 2003. [DSTE-PROTO] F. Le Faucheur, "Protocol extensions for support of Diff-Serv-aware MPLS Traffic Engineering," draft-ietf-tewg-diff-te- proto-05.txt, September 2003. [BC-RD] F. Le Faucheur, "Russian Dolls Bandwidth Constraints Model for Diff-Serv-aware MPLS Traffic Engineering," draft-ietf- tewg-diff-te-russian-01.txt, August 2003. [BC-MAM] W. Lai, "Bandwidth Constraint Models for Diffserv- aware MPLS Traffic Engineering," draft-wlai-tewg-bcmodel-03.txt, September 2003. [BC-MAR] J. Ash, "Max Allocation with Reservation BW Constraint Model for MPLS/DiffServ TE," draft-ietf-tewg-diff-te-mar-02.txt, October 2003. [SOFT-PREPT] M. R. Meyer, D. Maddux, and J.-P. Vasseur, "MPLS Traffic Engineering Soft preemption," draft-meyer-mpls-soft-preemption-00.txt, February 2003. [TEWG-FW] Awduche et al, "Overview and Principles of Internet Traffic Engineering," RFC3272, May 2002. [TE-REQ] Awduche et al, "Requirements for Traffic Engineering over MPLS," RFC2702, September 1999. [DEC-PREP] M. Peyravian and A. D. Kshemkalyani, "Decentralized Network Connection Preemption Algorithms," Computer Networks and ISDN Systems, vol. 30 (11), pp. 1029-1043, June 1998. [ATM-PREP] S. Poretsky and T. Gannon, "An Algorithm for Connection Precedence and Preemption in Asynchronous Transfer Mode (ATM) Networks," Proceedings of IEEE ICC 1998. [PREEMPTION] J. C. de Oliveira et al, "A New Preemption Policy for DiffServ-Aware Traffic Engineering to Minimize Rerouting," Proceedings of IEEE INFOCOM 2002. de Oliveira, Vasseur, Chen, and Scoglio 16 draft-deoliveira-diff-te-preemption-02.txt October, 2003 Jaudelice C. de Oliveira ECE Department Drexel University 3141 Chestnut Street Philadelphia, PA 19104 USA Email: jau@ece.drexel.edu Jean-Philippe Vasseur Cisco Systems, Inc. 300 Beaver Brook Road Boxborough , MA - 01719 USA Email: jpv@cisco.com Leonardo Chen Verizon Laboratories Network Architecture and Enterprise Technologies 40 Sylvan Rd. LA0MS55 Waltham, MA 02451 USA Email: leonardo.c.chen@verizon.com Caterina Scoglio Broadband and Wireless Networking Laboratory Georgia Institute of Technology 250 14th Street, Suite 556 Atlanta, GA 30318 USA Email: caterina@ece.gatech.edu de Oliveira, Vasseur, Chen, and Scoglio 17