B. R. Badrinath

Implementation and performance evaluation of Indirect TCP

With the advent of small portable computers and the technological advances in wireless communications, mobile wireless computing is likely to become very popular in the near future. Wireless links are slower and less reliable compared to wired links and are prone to loss of signal due to noise and fading. Furthermore, host mobility can give rise to periods of disconnection from the fixed network. The use of existing network protocols, which were developed mainly for the high bandwidth and faster wired links, with mobile computers thus gives rise to unique performance problems arising from host mobility and due to the characteristics of wireless medium. Indirect protocols can isolate mobility and wireless related problems using mobility support routers (MSRs) as intermediaries, which also provide backward compatibility with fixed network protocols. We present the implementation and performance evaluation of I-TCP, which is an indirect transport layer protocol for mobile wireless environments. Throughput comparison with regular (BSD) TCP shows that I-TCP performs significantly better in a wide range of conditions related to wireless losses and host mobility. We also describe the implementation and performance of I-TCP handoffs.

Semantics-based concurrency control: beyond commutativity

The concurrency of transactions executing on atomic data types can be enhanced through the use of semantic information about operations defined on these types. Hitherto, commutativity of operations has been exploited to provide enhanced concurrency while avoiding cascading aborts. We have identified a property known as recoverability which can be used to decrease the delay involved in processing non-commuting operations while still avoiding cascading aborts. When an invoked operation is recoverable with respect to an uncommitted operation, the invoked operation can be executed by forcing a commit-dependency between the invoked operation and the uncommitted operation; the transaction invoking the operation will not have to wait for the uncommitted operation to abort or commit. Further, this commit dependency only affects the order in which the operations should commit, if both commit; if either operation aborts, the other can still commit thus avoiding cascading aborts. To ensure the serializability of transactions, we force the recoverability relationship between transactions to be acyclic.

Performance evaluation of semantics-based multilevel concurrency control protocols

For next generation information systems, concurrency control mechanisms are required to handle high level abstract operations and to meet high throughput demands. The currently available single level concurrency control mechanisms for reads and writes are inadequate for future complex information systems. In this paper, we will present a new multilevel concurrency protocol that uses a semantics-based notion of conflict, which is weaker than commutativity, called recoverability. Further, operations are scheduled according to relative conflict, a conflict notion based on the structure of operations.nPerformance evaluation via extensive simulation studies show that with our multilevel concurrency control protocol, the performance improvement is significant when compared to that of a single level two-phase locking based concurrency control scheme or to that of a multilevel concurrency control scheme based on commutativity alone. Further, simulation studies show that our new multilevel concurrency control protocol performs better even with resource contention.

Synchronizing transactions on objects

A method is discussed for synchronizing operations on objects when the operations are invoked by transactions. The technique, which is motivated by a desire to make use of possible concurrency in accessing objects, takes into consideration the granularity at which operations affect an object. A dynamic method is presented for determining the compatibility of an invoked operation with respect to operations in progress. In making decisions, it utilizes the state of the object, the semantics of the uncommitted operations, the actual parameters of the invoked operation, and the effect of the operations on the objects. One of the attractive features of this technique is that a single framework can be used to deal with the problem of synchronizing access to simple objects as well as compound objects, i.e. objects in which some components are themselves objects. >

Semantics-based concurrency control: Beyond commutativity

The concurrency of transactions executing on atomic data types can be enhanced through the use of semantic information about operations defined on these types. Hitherto, commutativity of operations has been exploited to provide enhanced concurrency while avoiding cascading aborts. We have identified a property known as recoverability which can be used to decrease the delay involved in processing non-commuting operations while still avoiding cascading aborts. When an invoked operation is recoverable with respect to an uncommitted operation, the invoked operation can be executed by forcing a commit-dependency between the invoked operation and the uncommitted operation; the transaction invoking the operation will not have to wait for the uncommitted operation to abort or commit. Further, this commit dependency only affects the order in which the operations should commit, if both commit; if either operation aborts, the other can still commit thus avoiding cascading aborts. To ensure the serializability of transactions, we force the recoverability relationship between transactions to be acyclic.

Indirect Transport Layer Protocols for Mobile Wireless Environment

Internetworking protocols for mobile hosts have hitherto treated host mobility as a routing problem to be handled entirely within the network (IP) layer. Such an approach however, ignores the distinctive features of wireless mobile computing. IP-based transport protocols thus suffer from poor performance when used for communication between a mobile host and hosts on the wired network. This is caused by frequent disruptions in network layer connectivity due to mobility and wireless losses. We describe indirect transport layer protocols for mobile hosts which can tackle mobility and wireless related performance problems without compromising backward compatibility with the transport protocols used over the wired network. Indirect protocols utilize the resources of Mobility Support Routers (MSRs) to provide transport layer communication between mobile hosts and those on the fixed network. We also present performance figures for I-TCP, an indirect transport protocol for mobile computers that is compatible with TCP, showing substantial improvement in throughput over regular TCP.