Design and Analysis of Transport Protocols for Reliable High-Speed Communications

The design and analysis of transport protocols for reliable communications constitutes the topic of this dissertation. These transport protocols guarantee the sequenced and complete delivery of user data over networks which may lose, duplicate and reorder packets. Reliable transport services are required by a wide range of applications such as the World-Wide Web, remote network access, and distributed computing. The design of these protocols is heavily influenced by the parameters of the underlying network infrastructure and by the assumptions about the host computers and applications. Therefore the recent advances in optical transmission and computer technologies stimulated the design of several novel transport protocols. Many of the proposed protocols use similar or at least related techniques. Our goal with this thesis is to improve the understanding of reliable communications by analyzing the protocols that implement this service and to contribute to the design of reliable transport protocols. The basis of our analysis is the formal specification and verification of the protocol mechanisms under investigation. The behavior of the protocol is captured by a state-transition system and properties are established using assertional reasoning. The framework is capable to handle unbounded and modulo-N state variables and to capture real-time aspects of the protocols which is essential for the modeling of realistic systems. Practical protocols of considerable complexity are specified and verified in the thesis. One advantage of the formal verification is that it increases our confidence in the correctness of these protocols. The formalism forces us to clarify all the details of the working of the protocol and to state explicitly every assumption about the protocol and its environment. During the process of the verification one also gains insight into the mechanisms of the protocol. But probably the most important result is that during the verication we obtain conditions for the correctness of the protocol in the form of inequalities on some protocol parameters. These conditions allow the comparison of the different protocol mechanisms and can be used to judge the suitability of a protocol for a certain environment. The functionality of transport protocols can be naturally divided into data transfer and connection management. Data transfer deals with the sequenced delivery of user data, while connection management is concerned with the orderly setup and release of connections.\ud In the thesis we study three different data transfer protocols. The usage of timestamps in data transfer protocols is analyzed in detail through the example of the PAWS mechanism which was proposed as an extension to TCP. The analysis reveals that the use of timestamps increases the functionality of the transport protocol by facilitating the simple measurement of round-trip delays, but it also reduces the maximum allowable transmission rate as compared to the plain sliding-window protocol. Another data transfer protocol called SNR is analyzed which is based on the idea of periodic state exchange. We start from an earlier specification of SNR and compare it to the plain sliding-window protocol. The analysis reveals that the maximum transmission speed achievable by that SNR specification is higher than that of the plain sliding-window protocol, but it comes with a serious limitation. In the SNR specication it is assumed that no duplicates are generated by either the network or the transport protocol itself. This assumption may seriously limit the eective performance of the protocol in case of losses in the network and demonstrates the importance of considering all the assumptions when selecting a protocol for a certain environment. The use of timestamps is also investigated in the context of connection management protocols. The detailed analysis of the connection setup protocol SCMP is presented which is based on the assumption that clocks of computers can be synchronized relatively cheaply even in a large network. In our verification it is proven that the safety of the protocol does not depend of the synchronization assumption, therefore the protocol can be used safely in cases when there are no absolute guarantees of the clocks being synchronized. Since practical clock synchronization algorithms give only probabilistic guarantees, our result provides an important theoretical support of the applicability of the protocol in practical environments. Based on earlier work by others, a family of connection management protocols is analyzed that use a cache to store information needed to shorten the connection setup latency. We contribute to this work by proposing improvements which allow to reduce considerably the memory usage of these protocols. Furthermore, we show that the correctness of the protocol can be assured without assuming an upper bound on the incarnation lifetime, i.e., the maximum duration of a connection. This result greatly improves the practical applicability of the protocol

NEEM: network-friendly epidemic multicast

Epidemic, or probabilistic, multicast protocols have emerged as a viable mechanism to circumvent the scalabil- ity problems of reliable multicast protocols. However, most existing epidemic approaches use connectionless transport protocols to exchange messages and rely on the intrinsic robustness of the epidemic dissemination to mask network omissions. Unfortunately, such an approach is not network- friendly, since the epidemic protocol makes no effort to re- duce the load imposed on the network when the system is congested. In this paper, we propose a novel epidemic protocol whose main characteristic is to be network-friendly. This property is achieved by relying on connection-oriented transport connections, such as TCP/IP, to support the com- munication among peers. Since during congestion mes- sages accumulate in the border of the network, the pro- tocol uses an innovative buffer management scheme, that combines different selection techniques to discard messages upon overflow. This technique improves the quality of the information delivered to the application during periods of network congestion. The protocol has been implemented and the benefits of the approach are illustrated using a com- bination of experimental and simulation results

Computing infrastructure issues in distributed communications systems : a survey of operating system transport system architectures

The performance of distributed applications (such as file transfer, remote login, tele-conferencing, full-motion video, and scientific visualization) is influenced by several factors that interact in complex ways. In particular, application performance is significantly affected both by communication infrastructure factors and computing infrastructure factors. Several communication infrastructure factors include channel speed, bit-error rate, and congestion at intermediate switching nodes. Computing infrastructure factors include (among other things) both protocol processing activities (such as connection management, flow control, error detection, and retransmission) and general operating system factors (such as memory latency, CPU speed, interrupt and context switching overhead, process architecture, and message buffering). Due to a several orders of magnitude increase in network channel speed and an increase in application diversity, performance bottlenecks are shifting from the network factors to the transport system factors.This paper defines an abstraction called an "Operating System Transport System Architecture" (OSTSA) that is used to classify the major components and services in the computing infrastructure. End-to-end network protocols such as TCP, TP4, VMTP, XTP, and Delta-t typically run on general-purpose computers, where they utilize various operating system resources such as processors, virtual memory, and network controllers. The OSTSA provides services that integrate these resources to support distributed applications running on local and wide area networks.A taxonomy is presented to evaluate OSTSAs in terms of their support for protocol processing activities. We use this taxonomy to compare and contrast five general-purpose commercial and experimental operating systems including System V UNIX, BSD UNIX, the x-kernel, Choices, and Xinu

A Survey on Handover Management in Mobility Architectures

This work presents a comprehensive and structured taxonomy of available techniques for managing the handover process in mobility architectures. Representative works from the existing literature have been divided into appropriate categories, based on their ability to support horizontal handovers, vertical handovers and multihoming. We describe approaches designed to work on the current Internet (i.e. IPv4-based networks), as well as those that have been devised for the "future" Internet (e.g. IPv6-based networks and extensions). Quantitative measures and qualitative indicators are also presented and used to evaluate and compare the examined approaches. This critical review provides some valuable guidelines and suggestions for designing and developing mobility architectures, including some practical expedients (e.g. those required in the current Internet environment), aimed to cope with the presence of NAT/firewalls and to provide support to legacy systems and several communication protocols working at the application layer

Connection Robustness for Wireless Moving Networks Using Transport Layer Multi-homing

Given any form of mobility management through wireless communication, one useful enhancement is improving the reliability and robustness of transport-layer connections in a heterogeneous mobile environment. This is particularly true in the case of mobile networks with multiple vertical handovers. In this thesis, issues and challenges in mobility management for mobile terminals in such a scenario are addressed, and a number of techniques to facilitate and improve efficiency and the QoS for such a handover are proposed and investigated. These are initially considered in an end-to-end context and all protocols and changes happened in the middleware of the connection where the network is involved with handover issues and end user transparency is satisfied. This thesis begins by investigating mobility management solutions particularly the transport layer models, also making significant observation pertinent to multi-homing for moving networks in general. A new scheme for transport layer tunnelling based on SCTP is proposed. Consequently a novel protocol to handle seamless network mobility in heterogeneous mobile networks, named nSCTP, is proposed. Efficiency of this protocol in relation to QoS for handover parameters in an end-to-end connection while wired and wireless networks are available is considered. Analytically and experimentally it has been proved that this new scheme can significantly increase the throughput, particularly when the mobile networks roam frequently. The detailed plan for the future improvements and expansion is also provided

Reflections on security options for the real-time transport protocol framework

The Real-time Transport Protocol (RTP) supports a range of video conferencing, telephony, and streaming video ap- plications, but offers few native security features. We discuss the problem of securing RTP, considering the range of applications. We outline why this makes RTP a difficult protocol to secure, and describe the approach we have recently proposed in the IETF to provide security for RTP applications. This approach treats RTP as a framework with a set of extensible security building blocks, and prescribes mandatory-to-implement security at the level of different application classes, rather than at the level of the media transport protocol