Krishan K. Sabnani

Reliable multicast transport protocol (RMTP)

This paper presents the design, implementation, and performance of a reliable multicast transport protocol (RMTP). The RMTP is based on a hierarchical structure in which receivers are grouped into local regions or domains and in each domain there is a special receiver called a designated receiver (DR) which is responsible for sending acknowledgments periodically to the sender, for processing acknowledgment from receivers in its domain, and for retransmitting lost packets to the corresponding receivers. Since lost packets are recovered by local retransmissions as opposed to retransmissions from the original sender, end-to-end latency is significantly reduced, and the overall throughput is improved as well. Also, since only the DRs send their acknowledgments to the sender, instead of all receivers sending their acknowledgments to the sender, a single acknowledgment is generated per local region, and this prevents acknowledgment implosion. Receivers in RMTP send their acknowledgments to the DRs periodically, thereby simplifying error recovery. In addition, lost packets are recovered by selective repeat retransmissions, leading to improved throughput at the cost of minimal additional buffering at the receivers. This paper also describes the implementation of RMTP and its performance on the Internet.

A protocol test generation procedure

Abstract A procedure presented here generates test sequences for checking the conformity of an implementation to the control portion of a protocol specification, which is modeled as a deterministic finite-state machine (FSM). A test sequence generated by the procedure given here tours all state transitions and uses a unique signature for each state, called the Unique Input/Output (UIO) sequence. A UIO sequence for a state is an input/output behavior that is not exhibited by any other state. An algorithm is presented for generating a minimum-length UIO sequence, should it exist, for a given state. UIO sequences may not exist for some states.

AIRMAIL: a link-layer protocol for wireless networks

This paper describes the design and performance of a link-layer protocol for indoor and outdoor wireless networks. The protocol is asymmetric to reduce the processing load at the mobile, reliability is established by a combination of automatic repeat request and forward error correction, and link-layer packets are transferred appropriately during handoffs. The protocol is namedAIRMAIL (AsymmetrIc Reliable Mobile Access In Link-layer). The asymmetry is needed in the design because the mobile terminals have limited power and smaller processing capability than the base stations. The key ideas in the asymmetric protocol design consist of placing bulk of the intelligence in the base station as opposed to placing it symmetrically, in requiring the mobile terminal to combine several acknowledgments into a single acknowledgment to conserve power, and in designing the base stations to send periodic status messages, while making the acknowledgment from the mobile terminal eventdriven. The asymmetry in the protocol design results in a one-third reduction of compiled code. The forward error correction technique incorporates three levels of channel coding which interact adaptively. The motivation for using a combination of forward error correction and link-layer retransmissions is to obtain better performance in terms of end-to-end throughput and latency by correcting errors in an unreliable wireless channel in addition to end-to-end correction rather than by correcting errors only by end-to-end retransmissions. The coding overhead is changed adaptively so that bandwidth expansion due to forward error correction is minimized. Integrity of the link during handoffs (in the face of mobility) is handled by window management and state transfer. The protocol has been implemented. Experimental performance results based on the implementation are presented.

Design and implementation of a high-speed transport protocol

The design, analysis, and implementation of an end-to-end transport protocol that is capable of high throughput consistent with the evolving high-speed physical networks based on fiber-optic transmission lines and high-capacity switches are presented. Unlike current transport protocols in which changes in control/state information are exchanged between the two communicating entities only when some significant event occurs, this protocol exchanges relevant and full state information periodically and frequently. It is shown that this reduces the complexity of protocol processing by removing many of the procedures required to recover from network inadequacies such as bit errors, packet loss, and out-of-sequence packets and makes it more amenable to parallel processing. Also, to increase channel utilization in the presence of high-speed, long-latency networks and to support diagrams, and efficient implementation of the selective repeat method of error control is incorporated in the protocol. An implementation using a Motorola 68030-based multiprocessor as a front-end processor is described. The current implementation can comfortably handle 10-15 kpackets/s. >

Multicast Scheduling in Cellular Data Networks

Multicast is an efficient means of transmitting the same content to multiple receivers while minimizing network resource usage. Applications that can benefit from multicast such as multimedia streaming and download, are now being deployed over 3G wireless data networks. Existing multicast schemes transmit data at a fixed rate that can accommodate the farthest located users in a cell. However, users belonging to the same multicast group can have widely different channel conditions. Thus existing schemes are too conservative by limiting the throughput of users close to the base station. We propose two proportional fair multicast scheduling algorithms that can adapt to dynamic channel states in cellular data networks that use time division multiplexing: Inter-group Proportional Fairness (IPF) and multicast proportional fairness (MPF). These scheduling algorithms take into account (1) reported data rate requests from users which dynamically change to match their link states to the base station, and (2) the average received throughput of each user inside its cell. This information is used by the base station to select an appropriate data rate for each group. We prove that IPF and MPF achieve proportional fairness among groups and among all users in a group inside a cell respectively. Through extensive packet-level simulations, we demonstrate that these algorithms achieve good balance between throughput and fairness among users and groups.

Formal methods for generating protocol conformance test sequences

The four major methods of conformance test generation reported in the literature are reviewed: transition tours; distinguishing sequences; characterizing sequences; and unique input/output sequences. These methods are used to test the control portion of a protocol specification. The conformance testing concepts developed in the standards world are summarized. Their relationship with the four formal methods is discussed. >

Conformance testing of protocols specified as communicating finite state machines-a guided random walk based approach

We present a new approach for conformance testing of protocols specified as a collection of communicating finite state machines (FSMs). Our approach uses a guided random walk procedure. This procedure attempts to cover all transitions in the component FSMs. We also introduce the concept of observers that check some aspect of protocol behavior. We present the result of applying our method to two example protocols: full-duplex alternating bit protocol and the ATM-adaptation-layer-convergence protocol. Applying our procedure to the ATM adaptation layer, 99% of component FSMs edges can be covered in a test with 11692 input steps. Previous approaches cannot do conformance test generation for standard protocols (such as asynchronous transfer mode (ATM) adaptation layer) specified as a collection of communicating FSMs.

Multicast transport protocols for high speed networks

This paper presents the design and analysis of three reliable multicast transport protocols for high speed networks. The novelty of these protocols lies in the technique used in combining the acknowledgments of individual destinations along the underlying multicast tree to prevent acknowledgement implosion and in the technique used in preventing unnecessary retransmission by performing local multicasts. These protocols use the periodic exchange of complete state information between the source and the destinations and a block-based Selective Repeat retransmission scheme to improve the overall performance in a high speed networking environment. Performance of each protocol is analyzed in terms of throughput, end-to-end delay, buffer requirement, acknowledgment traffic and retransmission traffic. Based on this analysis and the complexity of implementation, one of the three protocols is recommended for reliable multicasting in high speed networks.<<ETX>>

Challenges for nomadic computing: mobility management and wireless communications

In this paper, we present several challenges and innovative approaches to support nomadic computing. The nomadic computing environment is characterized by mobile users that may be connected to the network via wired or wireless means, many of whom will maintain only intermittent connectivity with the network. Furthermore, those accessing the network via wireless links will contend with limitations of the wireless media. We consider three general techniques for addressing these challenges: (1) asymmetric design of applications and protocols, (2) the use of network-based proxies which perform complex functions on behalf of mobile users, and (3) the use of pre-fetching and caching of critical data. We examine how these techniques have been applied to several systems, and present results in an attempt to quantify their relative effectiveness.

An algorithmic procedure for checking safety properties of protocols

A procedure for checking safety properties of communication protocols is presented. A protocol is specified as a collection of communicating finite-state machines (FSMs). Two novel algorithms used in this procedure are described. The first algorithm does incremental composition and reduction of FSMs. It uses three heuristic rules which reduce the number of states in the global FSM by one to two orders of magnitude while maintaining its observational equivalence. The second algorithm checks whether the behavior of one FSM is a subset of another FSMs behavior. This procedure has been applied to the ISDN Q.931 and alternating bit protocols. >