CAM: Courteous bandwidth constraints allocation model

CAM: Courteous Bandwidth Constraints Allocation Model Chafika Tata

Michel Kadoch

Electric department École de technologie supérieure Montreal, Canada

Electric department École de technologie supérieure Montreal, Canada

Abstract— This paper proposes a new Bandwidth Constraints Model for MPLS networks, called CAM (Courteous Allocation Model). CAM allocates added bandwidth to lower classes of traffic without affecting the high priority traffic. Simulations are performed to compare CAM with the two basic Constraints Models, namely, MAM (Maximum Allocation Model) and RDM (Russian Dolls Model). The results show that our model gives significant improvement in terms of delay, throughput and packets loss. Keywords—MAM; RDM; CAM;QoS

I.

INTRODUCTION

MPLS as defined by IETF(Internet Engineering TaskForce) is based on the principal of label switching in order to specify the routing path from source to destination. Unlike IP protocol, MPLS is connexion oriented. A signalling protocol is applied first to define the path that has to be used by the packets. One such protocol is LDP (Label Distribution Protocol). Every MPLS router stores a switching table which includes information related the packet entry port, its destination address, its entry label and the exit label. One of the objectives of MPLS is to improve the performance of the packet forwarding by deciding on the path to be followed towards the destination. This is the LSP (Label Switched Path). However, different requirement in term of QoS in relation to different type of traffic had introduced an important mechanism that manages bandwidth resources, the traffic engineering (TE). The application of TE enables the possibility of defining additional paths in order to share resources and thus prevent congestion in the network. The new selected paths are not necessary the least cost ones. They can, however, improve drastically the network performance. Because the needs and the requirements of the data flow circulating on the network are not the same, the application of the MPLS-TE cannot successfully attain the objective of the best QoS of each service constituting the data. The DiffServ (Differentiated Services) protocol is applied to respond to this need, thus forming DS-TE (DiffServ-aware MPLS Traffic Engineering). DiffServ is a mechanism that allows guarantee of QoS for each type of traffic according to the class of service it belongs to. It ensures the management and allocation of available bandwidth in the network. There

are however class of service constraints in terms of required bandwidth to maintain the appropriate QoS. One of the key mechanisms of the DS-TE (DiffServ-aware MPLS traffic engineering), is the specification of a bandwidth constraint (BC) model, which describes the allocation the bandwidth to individual class types (CTs). [1]The maximal number of CTs is 8 (CT0 to CT7), and the best-effort service is mapped to CT0. DS-TE introduces the following new concepts: • Class-type (CT) which is a group of traffic trunks based on their QoS values so that they share the same bandwidth reservation, and a single class-type can represent one or more classes[2] • Bandwidth Constraint (BC) is a limit on the percentage of a links bandwidth that a particular class-type can use [2] Several Bandwidth Constraints models are proposed in the literature for the DS-TE scheme. The basic ones are Maximum Allocation Model (MAM) [3] and the Russia Dolls Allocation Model (RDM) [4]. Other schemes are realized to improve the performance of the MAM and RDM mechanisms, such as the Generalized Russian Dolls Model, G-RDM [5] and Self-adaptive bandwidth constraints model SAM[1]. The main objective of these models is to guarantee a better QoS for the real-time class of service such as voice and video. The bandwidth allocation follows a differentiation policy for packets with high priority with respect to others. In order for the differentiation not to be too harsh on lower priority traffic, it would be pertinent to establish rules that would favour lower priority traffic when conditions allow it. This would be based on the fact that the reserved bandwidth for high priority classes can be underutilized when applying these models. This would defeat the objective of a reliable and efficient management of bandwidth that should otherwise guarantee the QoS performance. This paper proposes a new mechanism for Bandwidth Allocation Model named CAM (Courteous Allocation Model). The main objective of CAM is to assure the allocation of the bandwidth in the network, by offering a higher priority level for otherwise non favoured traffic. This privilege is offered only if the QoS of higher priority traffic allows it. This required condition is sufficient and can be represented as follows:  The rate of loss of high priority packets is less than the tolerated packet loss threshold.



B. The RDM model:

The introduction of DiffServ in MPLS-TE improves the performance of networks with managing efficiently bandwidth allocation and considering the constraints imposed by each class of service. The different Bandwidth Constraints Allocation Models have the objective of reducing the delays and the loss of packets for high priority classes such as voice and video. This is in effect the task of DS-TE. In DS-TE architectures, a maximum of 8 ClassType (CT) [6] are designed, from CT0 to CT7. Many BC models are proposed to perform the bandwidth allocation in DS-TE networks.

The second BC scheme is RDM. According to [4] where 8 CTs are active, the RDM Bandwidth Constraints can also be expressed in the following way: - All LSPs from CT7 use no more than BC7 - All LSPs from CT6 and CT7 use no more than BC6 - All LSPs from CT5, CT6 and CT7 use no more than BC5 …etc. - All LSPs from CT0, CT1, ..., CT7 use no more than BC0 = "Maximum Reservable Bandwidth". Contrary to MAM, RDM is different by the fact that the sum of bandwidth that can be reserved by active CTj classes, where j ϵ {0, K-1}, cannot exceed the value of the Bandwidth Constraints BCi of the CTi class, is being the range of the smallest active class. In other words, i corresponds to the number of the lowest priority class (formula 4). Otherwise, this upper bound BCi which cannot be exceeded, is delimited by M. M is the maximum Bandwidth reserveable value. The other conditions are the same as MAM. RDM is defined as follows: [8]

The rate of loss of lower priority packets is over the tolerated packet loss threshold. In Other words, CAM should improve the lower priority service without affecting the higher priority traffics. The rest of this paper is organized as follows. Section II is an overview of the principal BC models. Section III describes a new BC framework, called the Courteous Allocation model, CAM. Whereas section IV discusses the different simulations and section V concludes this work. II.

BANDWIDTH CONSTRAINT ALLOCATION MODEL

A. The MAM model: MAM [3] is the first model realized as a BC scheme. It assigns bandwidth constraints to each CT[2]. Each class of service CT is allocated a fixed bandwidth. This bandwidth cannot be used by any other CT even if this bandwidth is unused. The MAM is defined as follows: All the active CT classes share the available bandwidth. Each CTi class can reserve a specific bandwidth quantity up to the value Ni. Note that Ni cannot exceed BCi (formula 1, representing the Bandwidth Constraints relative to the class 1. BCi cannot go over the maximum possible reserved bandwidth (formula 1) represented by M. On the other hand, the sum total of all reserved bandwidth for different active CTi classes, when i ϵ {0, K-1} cannot be higher than the value of M (formula 2). However, the sum of BCi for i ϵ {0, K-1} can go beyond the threshold M (formula 3). These MAM characterising are represented by the following formulas: 1.

For each i  {0, K-1}

1.

C-1

 Nj  BCi  M Where M is the maximum possible reserved bandwidth; 2. with the constraint C-1

 Ni  M

(5)

i=0

C. The G-RDM model: G-RDM is another BC approach. It’s an improvement of the RDM mechanism. The basic of the G-RDM is a mixture of both of MAM and RDM. G-RDM allows bandwidth to be shared among different CTs (common pool) like RDM. On other hand, it guarantees a predefined amount of bandwidth to each of them (private pool) like MAM [5] This model is defined as follows: For each CTi C-1

(1)

 (ni+mi)  BCj M

(6)

i=j

Where K is the number of active CT, Ni is the reserved Bandwidth, BCi is the CTi Bandwidth Constraints and M is the maximum reservable bandwidth. 2.

(4)

j=i

1. Ni  BCi M

For each i  {0, N-1}

with the constraint C-1

 Ni  M

(7)

i=0

with the constraint

2.

Finally

K-1

 Ni  M

(2)

0

3.

C-1

 BCi  M

(8)

0

Finally K-1

 BCi  M 0

(3)

The G-RDM conditions differ from RDM only for the first constraint (formula 6). This later requires that the sum of possible reserved bandwidth is less than the Bandwidth Constraint of the lowest class. This sum includes the

bandwidths belonging to two pools, namely the private pool and the common pool. While explaining these different approaches, it is noticed that priority classes are always the first beneficiaries of the bandwidth. On the other hand, these models use a positive discrimination between classes. However, they do not give full satisfaction to any class, especially lower classes. The equity these methods try to attain became increasingly difficult as traffic of lower priority keeps on increasing. An allocated bandwidth will be held by upper classes even if not used. It is therefore conceivable to design a model that will distribute the underutilized bandwidth in favour of lower priority traffic, without affecting the QoS of other classes. III.

CAM: COURTEOUS ALLOCATION MODEL

The different Bandwidth Allocation Constraints models proposed [3-5] favour higher priority classes to the detriment of lower classes. This discrimination creates an underutilization of the bandwidth reserved for privilege classes if their traffic is not important. The proposed solution, CAM, exploits the residual bandwidth in favour of lower classes, thus rendering the distribution of the resource more equitable. Note that no additional bandwidth will be allocated to lower priority class, if it affects in any way the QoS of upper classes. As the RDM model, the Courteous Allocation Model (CAM) allows the bandwidth sharing between CTs. But unlike the RDM, a part of a reserveable bandwidth of CTi may be left for the CTi-1 if conditions cited in [8] are respected. The outstanding feature that distinguishes CAM from RDM is the possibility to give up a given bandwidth allocated to a CTi class for the benefit of a class with less priority CT(i-1) provided the QoS is maintained. Thus every N(i-1) bandwidth of CT(i-1) class is composed of two bandwidth quantities Ni and N(i-1) (formula 11). N’i-1 is the total possible reserved bandwidth for the CTi-1 class. N’i-1 is the amount of bandwidth that can be released by the CTi class for the CTi-1 one, and the Ni-1 is the minimum bandwidth that the CTi class needs. This model is defined as follows 1. For each i  {0, N-1} C-1

 Nj  BCi  M

(9)

2.

with the constraint C-1

 Ni  M

and 3. each i= 1, C N’i-1 = N’i+Ni-1 4.

BC1

BC0

BC’0

N1 (CT1) N0 + N1

N0’ (CT’0) (CT0+CT1)

Fig. 1. Bandwidth allocation in CAM

Where M is the maximum reservable bandwidth;

(11)

Finally C-1

 BCi  M

(12)

0

IV.

SIMULATION

The Bandwidth Allocation model mechanism is simulated using a queuing scheme model (figure 2). Our model includes four queues to simulate the Bandwidth Constraints Model behaviour. The two first ones allow the simulation of the first condition of the model. The two others simulate the second condition of the MAM, RDM and CAM models. In Priority Queuing (PQ), the voice queue which has the higher priority is processed earlier than FTP queue. In Courteous Priority Queuing (CPQ), packets from the FTP queue may be processed before their rounds if the Voice Quality of Service tolerates it according to the voice packets loss rate. Simulation parameters are included in table 1. Reference [8] gives more information about the CPQ Queuing Model and the thresholds parameters (Th_Q1Q and Th_Q2Q for this study). The simulation parameters are the same for the tree models. However, the CAM model uses two additional parameters, namely the voice queue fill up threshold Th_Q1Q, and the FTP queue fill up threshold Th_Q2Q. These two parameters are essential for the simulations because the conditions applied on the CAM algorithm are based on the filling up rate of the two queues voice and FTP. It is important to note that the courtesy is applied only if the size of the FTP queue goes over the TH_Q2Q threshold and the size of the voice queue has not reached the threshold Th_Q1Q. It is important to note that the threshold Th_Q1Q is determined in relation to the voice traffic quantity. Indeed, as this traffic is less important, the more Q1: Voice

Q1Q: Voice

Q2: FTP

Q2Q: FTP

j=i

BC’1

(10)

i=0

PQ or CPQ

Fig. 2. Queuing Model

Th_Q1Q increases. The objective is to keep the queue with a level of voice packets null or acceptable, and assure an end to end delay less or equal to 200 ms. Table 2 summarizes the most important numerical results for the three models CAM, RDM and MAM. The information included in the table shows that the MAM model assures the best mean waiting time for the voice. On the other hand, the results demonstrate that not only CAM model enhances the mean waiting time for the data, but it

reduces the packet loss rate. Although the number of packets lost increases, it is still acceptable. TABLE I.

SIMULATION PARAMETERS

Simulation Parameters Inter-arrival Voice (s) Inter-arrival FTP (s) Q1Q & Q2Q Service “” (s) Q2 service (s) Q1 service (s) Max Waiting Time allowed (Voice) (s) Q1_limite (packets) Q2_limite (packets) Q1Q_limite (packets) Q2Q_limite (packets) Th_Q1Q (packets) Th_Q2Q (packets) TABLE II.

CAM – RDM & MAM 0.0012 0.0008 0.002 0.002 0.001 0.02 120 150 150 150 50 130

Fig. 4. FTP delay (CAM, MAM & RDM)

Fig. 5. Voice Queue Length (CAM,MAM & RDM)

NUMERICAL RESULTS MAM

RDM

Simulation Time (s)

23.9

23.9

CAM 23.9

Voice mean delay (s)

0.000801

0.000829

0.017928

FTP mean delay (s)

0.11075

0.1053

0.05998

Packets arrived to Q1Q queue

20198

20032

20032

Packets lost from Q1Q queue

0

0

1515

Packets arrived to Q2Q queue

29783

29962

29962

Packets lost from Q2Q queue

2137

1871

307

Packets served with courteousy

18433

% of packets served witch courteousy

61.5213%

The numerical results shown in table 2 indicate the positive impact of the CAM algorithm in terms of delay improvement and reduction in FTP packets loss. The FTP delay is reduced 1.75 times with respect to RDM and 1.846 times with respect to MAM. The results with respect to the FTP packet loss are very encouraging for CAM which reduces the loss 6.09 times that of RDM and 6.96 times with respect to MAM. Furthermore, even with the loss of voice packets with CAM, the voice QoS is not degraded since it represents 7.56% off all voice packets transmitted. The main contribution of this solution is illustrated by the number of FTP packets that benefit from this courtesy.

Fig. 6. FTP Queue Length (CAM,MAM & RDM)

Fig. 7. FTP Packet Loss (CAM,MAM & RDM)

Our simulation results are shown in figures 3 to figure 8

Fig. 8. Voice Packet loss (CAM,MAM & RDM)

Fig. 3. Voice delay (CAM,MAM & RDM)

It is important to remind that the application of the courteous algorithm is applied if and only if the filling rate of the voice queue does not reach th_Q1Q. As this threshold is not reached, the rate of loss of voice packets (figure 8) is less than the tolerated one. Consequently, the voice packets lost following this application do not deteriorate the voice QoS since it stays below the tolerated voice packet loss. Figure 7 shows the contribution of our solution to reduce in a remarkable way the rate of packets loss for the FTP traffic. Furthermore, the number of packets lost in RDM and MAM is monotonically increasing with time, unlike the CAM which shows an almost stable packets loss. For the same range of time, figure 3 to figure 7 demonstrate that CAM respects the FTP quality of service without affecting the voice one.

REFERENCES [1] [2]

[3]

[4] [5] [6]

[7] [8]

V.

CONCLUSION

In this paper, we have proposed a new Bandwidth Constraints Model for MPLS networks, called CAM (Courteous Allocation Model). As the RDM model, the Courteous Allocation Model (CAM) allows the bandwidth sharing between CTs. But unlike the RDM, a part of a reservable bandwidth of CTi may be left for the CTi-1. The MAM model adapts a different behaviour for the bandwidth allocation. It specifies a fixed amount for each CTi. Because of this, a part of bandwidth will be underused if the amount of the voice traffic is low. The RDM model was modeled to enhance the use of this bandwidth, but it is still insufficient for performing the FTP quality of service. Our approach, namely the CAM, has improved the bandwidth allocation scheme for the FTP traffic without reducing the guarantied voice QoS in terms of delay and packets loss. The simulation results show that the CAM model gives a better performance compared to the RDM and the MAM models.

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