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
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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
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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
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.
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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,
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.
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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
|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).
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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
Abw(l)=0.
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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)
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
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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
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).
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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
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