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Versions: 00 01 02 03 04 05 06 RFC 4829

                                                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


         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

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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

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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

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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

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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


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


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


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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,


* 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.

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* 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


          v              NO

 Preemption Needed ? -------> Setup LSP/Update UB if a threshold is

          |                   crossed

          | YES


      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).

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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

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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.

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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].

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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


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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

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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)


      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

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      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


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


          (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


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).

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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).

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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


[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


Email: jau@ece.drexel.edu

Jean-Philippe Vasseur

Cisco Systems, Inc.

300 Beaver Brook Road

Boxborough , MA - 01719


Email: jpv@cisco.com

Leonardo Chen

Verizon Laboratories

Network Architecture and Enterprise Technologies

40 Sylvan Rd. LA0MS55

Waltham, MA 02451


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


Email: caterina@ece.gatech.edu

de Oliveira, Vasseur, Chen, and Scoglio                               17

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