Douglas E. Comer

Ubiquitous B-Tree

B-trees have become, de facto, a standard for file organization. File indexes of users, dedicated database systems, and general-purpose access methods have all been proposed and nnplemented using B-trees This paper reviews B-trees and shows why they have been so successful It discusses the major variations of the B-tree, especially the B+-tree, contrasting the relatwe merits and costs of each implementatmn. It illustrates a general purpose access method whmh uses a B-tree.

Conversation-based mail

A new message communication paradigm based on conversations that provides an alternative to memo- and conference-based mail is described. A conversation-based message system groups messages into conversations, and orders messages within a conversation according to the context in which they were written. The message context relation leads to an efficient implementation of conversations in a distributed environment and supports a natural ordering of messages when viewed by the user. Experience with a prototype demonstrates the workability of conversation-based mail and suggests that conversations provide a powerful tool for message communication.

The difficulty of optimum index selection

Given a file on a secondary store in which each record has several attributes, it is usually advantageous to build an index mechanism to decrease the cost of conducting transactions to the file. The problem of selecting attributes over which to index has been studied in the context of various storage structures and access assumptions. One algorithm to make an optimum index selection requires 2k steps in the worst case, where k is the number of attributes in the file. We examine the question of whether a more efficient algorithm might exist and show that even under a simple cost criterion the problem is computationally difficult in a precise sense. Our results extend directly to other related problems where the cost of the index depends on fixed values which are assigned to each attribute. Some practical implications are discussed.

The Complexity of Trie Index Construction

Trle structures are a convenient way of indexing files in which a key consists of a number of attributes Records correspond to leaves in the trle Retrieval proceeds by following a path from the root to a leaf, the choice of edges being determined by attribute values The size of a trle for a file depends on the order in which attributes are tested It is shown that determining minimal size tries IS an NP-complete problem for several variants of tries and that, for tries m which leaf chains are deleted, determining the trie for which average access time is minimal is also an NP-complete problem These results hold even for files in which attribute values are chosen from a binary or ternary alphabet KE] WORDS AND PHRASES reformation retrieval, trle indexes, trte size, average search t i m e , complexity CR CATEGORIES 3 74, 4 33, 5 25

The computer science research network CSNET: a history and status report

In 1981, the National Science Foundation started a five-year project totaling nearly

A rate-based congestion avoidance and control scheme for packet switched networks

5 million to construct a computer science research network, CSNET, connecting all groups engaged in computer science research. For an NSF division with an annual budget of

Analysis of a heuristic for full trie minimization

25 million, the award represents an unusual commitment to a single project; only a handful of such large awards have been made. What is CSNET? Why is it receiving such attention? How will it benefit the community? When will it be completed? Who are the architects and implementors?

Understanding naming in distributed systems

A congestion avoidance and control scheme that monitors the incoming traffic to each destination and provides rate-based feedback information to the sources of bursty traffic so that sources of traffic can adjust their packet rates to match the network capacity is described. The congestion avoidance mechanism at nodes on the periphery of the network controls incoming traffic so that it does not exceed the capacity of paths to different destinations. The congestion control mechanism at each node monitors the performance of adjacent links and generates rate control messages that warn the sources of traffic before congestion develops. Some existing schemes are reviewed, and the congestion avoidance and control scheme and its applicability to various transport protocols are discussed. Experiments show that the scheme is effective in preventing congestion inside the network and that it manages to restrict the traffic on any overloaded path to 80%-90% of its capacity.<<ETX>>

Heuristics for trie index minimization

A trie is a distributed-key search tree in which records from a file correspond to leaves in the tree. Retrieval consists of following a path from one root to a leaf, where the choice of edge at each node is determined by attribute values of the key. For full tries, those in which all leaves lie at the same depth, the problem of finding an ordering of attributes which yields a minimum size trie is NP-complete. This paper considers a “greedy” heuristic for constructing low-cost tries. It presents simulation experiments which show that the greedy method tends to produce tries with small size, and analysis leading to a worst case bound on approximations produced by the heuristic. It also shows a class of files for which the greedy method may perform badly, producing tries of high cost.

The NEBULA Future Internet Architecture

Naming in distributed systems is modelled as a string translation problem. Viewing names as strings and name resolution mechanisms as syntax directed translators provides a formal handle on the loosely understood concepts associated with naming: we give precise definitions for such informal terminology as name spaces, addresses, routes, source-routing, and implicit-routing; we identify the properties of naming systems, including under what conditions they support unique names, relative names, absolute names, and synonyms; and we discuss how the basic elements of the model can be implemented by name servers.