Sheila Fallon

An Analysis of Alterations to the SCTP RTO Calculation Mechanism for WLAN Environments

As a connection oriented transport layer protocol the Stream Control Transmission Protocol (SCTP) inherits many of the features of the Transmission Control Protocol (TCP) including the mechanism by which Retransmission Timeout (RTO) is calculated. Previous investigations have established that the mechanism through which SCTP calculates RTO is inappropriate in Wireless LAN (WLAN) environments. This paper investigates the performance implications of changes to the SCTP RTO calculation mechanism. In particular alterations to the parameters α, the smoothing factor, and β, the delay variance factor are investigated. Results indicate that performance improvements are achievable through careful selection of α and β values. Throughput improvements of 63% over the default mechanism defined in RFC 4960 are described. These performance improvements however, while significant, still can not address the switchover delays which result from the distortions caused by continuously increasing RTT values in WLAN environments.

Using 802.11 MAC Retransmissions for Path Selection in Multi-Homed Transport Layer Protocols

The Stream Control Transmission Protocol (SCTP) is a transport layer protocol which can support mobility through its multi-homing feature. One of the key parameters used by SCTP to manage path selection is Retransmission Time Out (RTO). We illustrate that SCTPs evaluation of underlying paths using end to end metrics, results in the calculation of excessive RTO for degraded wireless paths. This excessive RTO causes SCTP to behave in a counter intuitive manner which delays path switchover. Our investigation shows that the increased Round Trip Time (RTT) is as a result of retransmissions at the 802.11 MAC. We illustrate that when a connectionless 802.2 LLC service is utilized, 802.11 independently implements positive frame acknowledgement resulting in significantly increased RTT. We propose a cross layer path selection algorithm, which recognizes increased 802.11 MAC retransmissions as an indicator of imminent path failure. Results presented indicate that our cross layer approach has excellent performance in comparison to standard SCTP strategies.

An Adaptive Optimized RTO Algorithm for Multi-homed Wireless Environments

As a transport layer protocol SCTP uses end to end metrics, such as Retransmission Time Out (RTO), to manage mobility handover. Our investigation illustrates that Wireless LAN (WLAN) mobility causes continuously increased Round Trip Times (RTT) resulting from 802.11 MAC retransmissions, regardless of the service specified by upper layers. We present scenarios where the current understanding of SCTP switchover aggressiveness is invalid; spurious failovers together with excessive RTO result in new forms of receiver buffer blocking communication failure. Given wireless mobility performance issues, together with the ambiguity of end to end metrics, we propose an Adaptive Optimized RTO algorithm for wireless Access Networks (AORAN) which uses local as well as end to end metrics to manage mobility. AORAN measures RTT between the mobile node and Access Point (AP) to calculate wireless and Internet RTO subcomponents. We also show binary exponential backoff has negative effects on SCTP with increased wireless RTT; AORAN introduces a decision mechanism which implements backoff on RTO subcomponents only when appropriate.

CADMANT: Context Anomaly Detection for MAintenance and Network Troubleshooting

In telecommunications network troubleshooting, analytical applications are widely used. Such applications typically use CEP (Complex Event Processing) and SQL queries for data processing and network analysis. Performance engineers need in-depth knowledge of both the telecommunications domain and telecommunications data structures in order to create the required queries. Moreover valuable information contained in free form text data fields such as “additional_info”, “user_text” or “problem_text” can also be ignored. This work proposes CADMANT: Context Anomaly Detection for MAintenance and Network Troubleshooting. Traditional approaches focus on a specific record type and create specific cause and effect rules. With the CADMANT approach all free form text fields of alarms, logs, etc. are treated as text documents similar to Twitter feeds. CADMANT uses distance based outlier detection within sliding windows to detect abnormal terms at configurable time intervals. The CADMANT approach provides automated analysis without the requirement for SQL/CEP queries and provides distinct network insights in comparison to traditional approaches.

A Unified Notification Service Utilizing Explicit Wireless Degradation Notifications

The dynamic characteristics of heterogeneous networks require cross layer solutions to optimize performance. Many cross layer solutions however, have tightly coupled Upper Layer Protocols (ULP) to lower layer performance metrics. Such an approach hinders the introduction of potential performance enhancements. In this paper we propose a Unified Notification Service (UNS) which will enable cross layer solutions while decoupling ULPs from lower layer performance metrics. UNS is designed as an open extensible framework to accommodate both existing and emerging notification plug-ins. UNS will also enable the integration of existing protocols to form new notification types. We illustrate how the general purpose solution provided by UNS can incorporate a new Wireless Degradation Notification (WDN) plug-in. We compare the performance of MAC layer retransmissions and Received Signal Strength (RSS) as network performance indicators, and illustrate that a retransmission based metric outperforms an RSS based approach.