title: Failure resilience and traffic engineering for multi-controller software defined networking

authors: Zaicu, Nicu Florin

abstract: This thesis explores and proposes solutions to address the challenges faced by Multi-Controller SDN (MCSDN) systems when deploying TE optimisation on WANs. Despite the interest from the research community, existing MCSDN systems present limitations. For example, TE optimisation systems are computationally complex, have high consistency requirements, and need network-wide state to operate. Because of such requirements, MCSDN systems can encounter performance overheads and state consistency problems when implementing TE. Moreover, performance and consistency problems are more prominent when deploying the system on WANs as these network types have higher inter-device latency, delaying state propagation.

Unlike existing literature, this thesis presents several design choices that address all four challenges affecting MCSDN systems (scalability, consistency, resilience, and coordination). We use the presented design choices to build Helix, a hierarchical MCSDN system. Helix provides better scalability, performance and failure resilience compared to existing MCSDN systems by sharing minimal state between controllers, offloading operations closer to the data plane and deploying lightweight tasks.

A challenge that we faced when building Helix was that existing TE algorithms did not meet Helix's design choices. This thesis presents a new CSPF-based TE algorithm that needs minimal state to operate and supports offloading inter-area TE to local controllers, fulfilling Helix's requirements. Helix's TE algorithm provides better performance and forwarding stability, addressing 1.6x more congestion while performing up to 29x fewer path modifications than the other algorithms evaluated in our experiments.

While MCSDN literature has explored evaluating different aspects of system performance, there is a lack of readily available tools and concrete testing methodologies. To this end, this thesis provides concrete testing methodologies and tools readily available to the MCSDN community to evaluate the data plane failure resilience, control plane failure resilience, and TE optimisation performance of MCSDN systems.

text: Failure resilience and traffic engineering for multi-controller software defined networking This thesis explores and proposes solutions to address the challenges faced by Multi-Controller SDN (MCSDN) systems when deploying TE optimisation on WANs. Despite the interest from the research community, existing MCSDN systems present limitations. For example, TE optimisation systems are computationally complex, have high consistency requirements, and need network-wide state to operate. Because of such requirements, MCSDN systems can encounter performance overheads and state consistency problems when implementing TE. Moreover, performance and consistency problems are more prominent when deploying the system on WANs as these network types have higher inter-device latency, delaying state propagation.

Unlike existing literature, this thesis presents several design choices that address all four challenges affecting MCSDN systems (scalability, consistency, resilience, and coordination). We use the presented design choices to build Helix, a hierarchical MCSDN system. Helix provides better scalability, performance and failure resilience compared to existing MCSDN systems by sharing minimal state between controllers, offloading operations closer to the data plane and deploying lightweight tasks.

A challenge that we faced when building Helix was that existing TE algorithms did not meet Helix's design choices. This thesis presents a new CSPF-based TE algorithm that needs minimal state to operate and supports offloading inter-area TE to local controllers, fulfilling Helix's requirements. Helix's TE algorithm provides better performance and forwarding stability, addressing 1.6x more congestion while performing up to 29x fewer path modifications than the other algorithms evaluated in our experiments.

While MCSDN literature has explored evaluating different aspects of system performance, there is a lack of readily available tools and concrete testing methodologies. To this end, this thesis provides concrete testing methodologies and tools readily available to the MCSDN community to evaluate the data plane failure resilience, control plane failure resilience, and TE optimisation performance of MCSDN systems.

year: 2022

title: Improving the performance of Hierarchical Hidden Markov Models on Information Extraction tasks

authors: Chou, Lin-Yi

subjects: hidden Markov model

abstract: This thesis presents novel methods for creating and improving hierarchical hidden Markov models.  The work centers around transforming a traditional tree structured hierarchical hidden Markov model (HHMM) into an equivalent model that reuses repeated sub-trees.  This process temporarily breaks the tree structure constraint in order to leverage the benefits of combining repeated sub-trees. These benefits include lowered cost of testing and an increased accuracy of the final model-thus providing the model with greater performance.  The result is called a merged and simplified hierarchical hidden Markov model (MSHHMM).

The thesis goes on to detail four techniques for improving the performance of MSHHMMs when applied to information extraction tasks, in terms of accuracy and computational cost.  Briefly, these techniques are: a new formula for calculating the approximate probability of previously unseen events; pattern generalisation to transform observations, thus increasing testing speed and prediction accuracy; restructuring states to focus on state transitions; and an automated flattening technique for reducing the complexity of HHMMs.

The basic model and four improvements are evaluated by applying them to the well-known information extraction tasks of Reference Tagging and Text Chunking.  In both tasks, MSHHMMs show consistently good performance across varying sizes of training data. In the case of Reference Tagging, the accuracy of the MSHHMM is comparable to other methods. However, when the volume of training data is limited, MSHHMMs maintain high accuracy whereas other methods show a significant decrease.  These accuracy gains were achieved without any significant increase in processing time.  For the Text Chunking task the accuracy of the MSHHMM was again comparable to other methods. However, the other methods incurred much higher processing delays compared to the MSHHMM.  The results of these practical experiments demonstrate the benefits of the new method-increased accuracy, lower computation costs, and better performance.

text: Improving the performance of Hierarchical Hidden Markov Models on Information Extraction tasks This thesis presents novel methods for creating and improving hierarchical hidden Markov models.  The work centers around transforming a traditional tree structured hierarchical hidden Markov model (HHMM) into an equivalent model that reuses repeated sub-trees.  This process temporarily breaks the tree structure constraint in order to leverage the benefits of combining repeated sub-trees. These benefits include lowered cost of testing and an increased accuracy of the final model-thus providing the model with greater performance.  The result is called a merged and simplified hierarchical hidden Markov model (MSHHMM).

The thesis goes on to detail four techniques for improving the performance of MSHHMMs when applied to information extraction tasks, in terms of accuracy and computational cost.  Briefly, these techniques are: a new formula for calculating the approximate probability of previously unseen events; pattern generalisation to transform observations, thus increasing testing speed and prediction accuracy; restructuring states to focus on state transitions; and an automated flattening technique for reducing the complexity of HHMMs.

The basic model and four improvements are evaluated by applying them to the well-known information extraction tasks of Reference Tagging and Text Chunking.  In both tasks, MSHHMMs show consistently good performance across varying sizes of training data. In the case of Reference Tagging, the accuracy of the MSHHMM is comparable to other methods. However, when the volume of training data is limited, MSHHMMs maintain high accuracy whereas other methods show a significant decrease.  These accuracy gains were achieved without any significant increase in processing time.  For the Text Chunking task the accuracy of the MSHHMM was again comparable to other methods. However, the other methods incurred much higher processing delays compared to the MSHHMM.  The results of these practical experiments demonstrate the benefits of the new method-increased accuracy, lower computation costs, and better performance.

year: 2006

title: Challenges Faced – Implications for Policy: The Everyday Lives of Eastern European Women in New Zealand

authors: Ember, Adrienna

subjects: everyday life

abstract: This study explored the everyday lives, aspirations, and coping strategies of seven Eastern European immigrant women in New Zealand who came from Bulgaria, the former Eastern Germany, Hungary, Poland, Romania, Serbia, and Slovakia. The answers to the research questions aimed to contribute to the deconstruction of the invisibility and marginalisation of these women in New Zealand.
Guided by the philosophies of Kaupapa Māori Research; the theory of Human Wellbeing; and the theory of Positive Psychology, and using the methods of narrative inquiry, three interviews were conducted with each participant. The focus of the first interviews was to learn about the everyday lives of research participants in their countries of origin. The second interview explored the reasons of relocation to New Zealand, the settlement process, and how women re-established their lives in their new locations. In the final interviews women talked about their current everyday lives and their plans for the future.
Interviews were recorded and summarised in interview summary reports which research participants modified in collaboration with the researcher until a final version was achieved. In addition, women participated in two focus group sessions. The first session was devoted to establish connections among the participants and allow themes to emerge from their conversations. The second group session aimed to explore areas women struggled with in their lives the most: relationships and employment opportunities.
The study yielded contributions to the research topic, to the acculturation literature, and to the design of research with migrants. Firstly, it revealed both negative and positive aspects of the participants’ everyday lives and highlighted some under-researched cultural differences from the mainstream population.
Women highlighted as an important cultural difference their strong preference for straightforward communication which was often experienced as offensive and blatant by local New Zealanders. Research participants critiqued the necessity of networking to obtain jobs in New Zealand which is a clear obstacle for a newly arrived migrant.
This study also highlighted how the acculturation process is more individual and complex than conventional models have sought to explain. While traditional acculturation literature suggests that migrants go through some common patterns during their settlement that ideally leads to their assimilation to the host culture, in this research not all migrant women intended to assimilate. Indeed, those with a stronger wish to become part of the host culture reported more disappointments than those who embraced their cultural otherness as a positive aspect and did not mind reminding different from the dominant Pakeha culture.
Finally, for migrant studies a less common research design was applied by using the Kaupapa Māori framework. This philosophy proposes a research process that empowers immigrants to voice their needs and strengthen them as agents of their lives beyond the research process. While two strength based theories were used for data interpretation (the 3-D model of the Human Wellbeing Theory and the PERMA model of Positive Psychology), the data analysis revealed that both models showed some limitations by failing to incorporate the dimension of spiritual and physical wellbeing within their domains.

text: Challenges Faced – Implications for Policy: The Everyday Lives of Eastern European Women in New Zealand This study explored the everyday lives, aspirations, and coping strategies of seven Eastern European immigrant women in New Zealand who came from Bulgaria, the former Eastern Germany, Hungary, Poland, Romania, Serbia, and Slovakia. The answers to the research questions aimed to contribute to the deconstruction of the invisibility and marginalisation of these women in New Zealand.
Guided by the philosophies of Kaupapa Māori Research; the theory of Human Wellbeing; and the theory of Positive Psychology, and using the methods of narrative inquiry, three interviews were conducted with each participant. The focus of the first interviews was to learn about the everyday lives of research participants in their countries of origin. The second interview explored the reasons of relocation to New Zealand, the settlement process, and how women re-established their lives in their new locations. In the final interviews women talked about their current everyday lives and their plans for the future.
Interviews were recorded and summarised in interview summary reports which research participants modified in collaboration with the researcher until a final version was achieved. In addition, women participated in two focus group sessions. The first session was devoted to establish connections among the participants and allow themes to emerge from their conversations. The second group session aimed to explore areas women struggled with in their lives the most: relationships and employment opportunities.
The study yielded contributions to the research topic, to the acculturation literature, and to the design of research with migrants. Firstly, it revealed both negative and positive aspects of the participants’ everyday lives and highlighted some under-researched cultural differences from the mainstream population.
Women highlighted as an important cultural difference their strong preference for straightforward communication which was often experienced as offensive and blatant by local New Zealanders. Research participants critiqued the necessity of networking to obtain jobs in New Zealand which is a clear obstacle for a newly arrived migrant.
This study also highlighted how the acculturation process is more individual and complex than conventional models have sought to explain. While traditional acculturation literature suggests that migrants go through some common patterns during their settlement that ideally leads to their assimilation to the host culture, in this research not all migrant women intended to assimilate. Indeed, those with a stronger wish to become part of the host culture reported more disappointments than those who embraced their cultural otherness as a positive aspect and did not mind reminding different from the dominant Pakeha culture.
Finally, for migrant studies a less common research design was applied by using the Kaupapa Māori framework. This philosophy proposes a research process that empowers immigrants to voice their needs and strengthen them as agents of their lives beyond the research process. While two strength based theories were used for data interpretation (the 3-D model of the Human Wellbeing Theory and the PERMA model of Positive Psychology), the data analysis revealed that both models showed some limitations by failing to incorporate the dimension of spiritual and physical wellbeing within their domains.

year: 2016