title: Measuring Path MTU discovery behaviour

authors: Luckie, Matthew John

subjects: computer science

abstract: Path MTU Discovery (PMTUD) is widely believed to be unreliable because of firewalls that discard ICMP “Packet Too Big”messages. This paper measures PMTUD behaviour for 50,000 popular websites and finds the failure rate in IPv4 is much less than previous studies. We measure the overall failure rate between 5% and 18%, depending on the MTU of the constraining link. We explore methods webserver operators are using to reduce their dependence on PMTUD, and find 11% limit themselves to sending packets no larger than 1380 bytes. We identify a number of common behaviours that seem to be software bugs rather than filtering by firewalls. If these are corrected PMTUD failures could be reduced by 63%. We further find the IPv6 failure rate is less than the IPv4 rate even with more scope for failure in IPv6.

text: Measuring Path MTU discovery behaviour Path MTU Discovery (PMTUD) is widely believed to be unreliable because of firewalls that discard ICMP “Packet Too Big”messages. This paper measures PMTUD behaviour for 50,000 popular websites and finds the failure rate in IPv4 is much less than previous studies. We measure the overall failure rate between 5% and 18%, depending on the MTU of the constraining link. We explore methods webserver operators are using to reduce their dependence on PMTUD, and find 11% limit themselves to sending packets no larger than 1380 bytes. We identify a number of common behaviours that seem to be software bugs rather than filtering by firewalls. If these are corrected PMTUD failures could be reduced by 63%. We further find the IPv6 failure rate is less than the IPv4 rate even with more scope for failure in IPv6.

year: 2010

title: Placing guilt and shame: lone mothers' experiences of higher education in Aotearoa New Zealand

authors: Longhurst, Robyn

subjects: emotion

abstract: This study examines twelve lone mothers' experiences of higher education at two universities in New Zealand. Interviews illustrated that lone mothers are often constructed by society as people who deserve to feel guilt for burdening taxpayers and for raising children without a live-in father. This guilt is visceral. It is felt in and lived through bodies on a daily basis. For most of the interviewees, however, succeeding at university also prompted feelings of pride. Drawing on Elspeth Probyn's ideas about guilt, shame and pride enabled us to understand more about lone mothers' experiences of higher education. We remain less certain about guilt's affective capacity for politics in our particular case. For some mothers feeling guilt lead to a questioning of normative constructions of motherhood but for others this was not the case. It would seem, therefore, that guilt's transformative potential can be mobilized in both emancipatory and more normative ways. We also found that lone mothers engaged in higher learning occupy paradoxical space. Some become empowered to see their lives and relationships differently, from the margins and from the centre, through guilt and pride.

text: Placing guilt and shame: lone mothers' experiences of higher education in Aotearoa New Zealand This study examines twelve lone mothers' experiences of higher education at two universities in New Zealand. Interviews illustrated that lone mothers are often constructed by society as people who deserve to feel guilt for burdening taxpayers and for raising children without a live-in father. This guilt is visceral. It is felt in and lived through bodies on a daily basis. For most of the interviewees, however, succeeding at university also prompted feelings of pride. Drawing on Elspeth Probyn's ideas about guilt, shame and pride enabled us to understand more about lone mothers' experiences of higher education. We remain less certain about guilt's affective capacity for politics in our particular case. For some mothers feeling guilt lead to a questioning of normative constructions of motherhood but for others this was not the case. It would seem, therefore, that guilt's transformative potential can be mobilized in both emancipatory and more normative ways. We also found that lone mothers engaged in higher learning occupy paradoxical space. Some become empowered to see their lives and relationships differently, from the margins and from the centre, through guilt and pride.

year: 2012

title: Lake Rotokakahi water quality modelling

authors: Jones, Hannah Frances Emily

abstract: Lake Rotokakahi is an iwi-owned lake, administered by the Lake Rotokakahi Board of Control. The lake has a surface area of 4.4 km2, a mean depth of 17.5 m, and a catchment of 19.7 km2 of which most is in exotic forest (46.5%), with some pasture (27.8%) and regenerating native forest and scrub (25.7%). Lake Rotokakahi has only one permanent surface inflow and it is assumed that the lake must be predominantly groundwater fed. The lake outflow, Te Wairoa Stream, flows into Lake Tarawera. Monitoring indicates that water quality has recently declined in the lake, with a shift from a mesotrophic state in the 1990's, to currently being classified as eutrophic. Increased algal biomass has reduced water clarity and in May 2011 the first ever recorded algae bloom of a toxic cyanobacterium species resulted in a fish kill in Te Wairoa Stream. With the majority of the catchment in either exotic or native forest, the cause for the decline in water quality is unclear. However there is some concern that forestry harvesting operations may lead to increased surface runoff of sediment and associated phosphorus in ephemeral streams that feed into the lake. The objective of this study was to setup a water quality model for Lake Rotokakahi, which in the future may provide a decisionsupport tool for lake managers.
The model used, DYRESM-CAEDYM, is a one-dimensional (1D) coupled hydrodynamic-water quality model that has been widely used in New Zealand and overseas. Methodology for deriving model forcing data (e.g. meteorological data, inflow volumes and nutrient loads), including the application of catchment and lake water balances, is described in this report. The catchment water balance indicates that groundwater and rainfall account for 37% and 42%, respectively, of the total inflow volume to the lake, with surface and ephemeral inflows contributing a smaller proportion (c. 20% combined). Furthermore, it appears that groundwater contributes a significant proportion (c. 60 - 70%) of the total nutrient loads, although surface and ephemeral inflows also contribute a significant proportion (34%) of the total phosphorus load. It should be noted that the catchment water balance is based on a very limited dataset, and therefore subject to significant uncertainty. 
We simulated the period July 2009 to June 2012 with DYRESM-CAEDYM. Model performance statistics indicated reasonable simulation of water quality for the calibration period, but did not satisfactorily capture the magnitude and dynamics of chlorophyll a and some nutrient species over an independent model validation period. There may be a number of reasons for the poor model performance over the validation period. For example, it may be that zooplankton and/or freshwater mussels (kakahi), which were not included in the model configuration, may exert significant grazing pressure on the phytoplankton populations at certain times. Furthermore, the significant uncertainty in the catchment water and nutrient loads most likely will have affected model performance. Nutrient concentrations are routinely monitored only in the surface inflow. There are no measurements of nutrient and sediment loads in ephemeral streams, and there is very limited data on groundwater nutrient concentrations.
In its current form, the DYRESM-CAEDYM model is not suitable for scenario testing of lake management options. It is recommended that effort is directed at quantification of ephemeral and groundwater inflows and their associated nutrient and (particularly for ephemeral inflows) sediment loads. This may be particularly important for ephemeral inflows located close to the lake that may be influenced by forestry harvesting operations. Recent increases in measured total phosphorus and phosphate concentrations in the surface inflow and in the lake, combined with the degradation of water quality that has already occurred over the last 20 years, provides further impetus for addressing this critical information gap. Were new data to  become available, opportunities exist for improving the performance of the current model and for developing a three-dimensional coupled hydrodynamic-water quality model (e.g. ELCOM-CAEDYM) with representation of kakahi and/or zooplankton, which may be able to resolve questions pertinent to the functioning of the lake, and to lake and catchment management.

text: Lake Rotokakahi water quality modelling Lake Rotokakahi is an iwi-owned lake, administered by the Lake Rotokakahi Board of Control. The lake has a surface area of 4.4 km2, a mean depth of 17.5 m, and a catchment of 19.7 km2 of which most is in exotic forest (46.5%), with some pasture (27.8%) and regenerating native forest and scrub (25.7%). Lake Rotokakahi has only one permanent surface inflow and it is assumed that the lake must be predominantly groundwater fed. The lake outflow, Te Wairoa Stream, flows into Lake Tarawera. Monitoring indicates that water quality has recently declined in the lake, with a shift from a mesotrophic state in the 1990's, to currently being classified as eutrophic. Increased algal biomass has reduced water clarity and in May 2011 the first ever recorded algae bloom of a toxic cyanobacterium species resulted in a fish kill in Te Wairoa Stream. With the majority of the catchment in either exotic or native forest, the cause for the decline in water quality is unclear. However there is some concern that forestry harvesting operations may lead to increased surface runoff of sediment and associated phosphorus in ephemeral streams that feed into the lake. The objective of this study was to setup a water quality model for Lake Rotokakahi, which in the future may provide a decisionsupport tool for lake managers.
The model used, DYRESM-CAEDYM, is a one-dimensional (1D) coupled hydrodynamic-water quality model that has been widely used in New Zealand and overseas. Methodology for deriving model forcing data (e.g. meteorological data, inflow volumes and nutrient loads), including the application of catchment and lake water balances, is described in this report. The catchment water balance indicates that groundwater and rainfall account for 37% and 42%, respectively, of the total inflow volume to the lake, with surface and ephemeral inflows contributing a smaller proportion (c. 20% combined). Furthermore, it appears that groundwater contributes a significant proportion (c. 60 - 70%) of the total nutrient loads, although surface and ephemeral inflows also contribute a significant proportion (34%) of the total phosphorus load. It should be noted that the catchment water balance is based on a very limited dataset, and therefore subject to significant uncertainty. 
We simulated the period July 2009 to June 2012 with DYRESM-CAEDYM. Model performance statistics indicated reasonable simulation of water quality for the calibration period, but did not satisfactorily capture the magnitude and dynamics of chlorophyll a and some nutrient species over an independent model validation period. There may be a number of reasons for the poor model performance over the validation period. For example, it may be that zooplankton and/or freshwater mussels (kakahi), which were not included in the model configuration, may exert significant grazing pressure on the phytoplankton populations at certain times. Furthermore, the significant uncertainty in the catchment water and nutrient loads most likely will have affected model performance. Nutrient concentrations are routinely monitored only in the surface inflow. There are no measurements of nutrient and sediment loads in ephemeral streams, and there is very limited data on groundwater nutrient concentrations.
In its current form, the DYRESM-CAEDYM model is not suitable for scenario testing of lake management options. It is recommended that effort is directed at quantification of ephemeral and groundwater inflows and their associated nutrient and (particularly for ephemeral inflows) sediment loads. This may be particularly important for ephemeral inflows located close to the lake that may be influenced by forestry harvesting operations. Recent increases in measured total phosphorus and phosphate concentrations in the surface inflow and in the lake, combined with the degradation of water quality that has already occurred over the last 20 years, provides further impetus for addressing this critical information gap. Were new data to  become available, opportunities exist for improving the performance of the current model and for developing a three-dimensional coupled hydrodynamic-water quality model (e.g. ELCOM-CAEDYM) with representation of kakahi and/or zooplankton, which may be able to resolve questions pertinent to the functioning of the lake, and to lake and catchment management.

year: 2014