Abstract. Scenario languages based on Message Sequence Charts (MSCs) and related notations have been widely studied in the last decade [14, 13, 2, 9, 6, 12, 8]. The high expressive power of scenarios renders many basic problems concerning these languages undecidable. The most expressive class for which several problems are known to be decidable is one which possesses a behavioral property called “existentially bounded”. However, scenarios outside this class are frequently exhibited by asynchronous distributed systems such as sliding window protocols. We propose here an extension of MSCs called Causal Message Sequence Charts, which preserves decidability without requiring existential bounds. Interestingly, it can also model scenarios from sliding window protocols. We establish the expressive power and complexity of decision procedures for various subclasses of Causal Message Sequence Charts. 1

Abstract—We revisit the problem of connection management for reliable transport as part of our clean-slate Recursive Inter-Net Architecture (RINA) [5]. At one extreme, a pure soft-state (SS) approach (as in Delta-t [15]) safely removes the state of a connection at the sender and receiver once the state timers expire without the need for explicit removal messages. And new connections are established without an explicit handshaking phase. On the other hand, a hybrid hard-state/soft-state (HS+SS) approach (as in TCP) uses both explicit handshaking as well as more limited timer-based management of the connection’s state. In this paper, we consider the worst-case scenario of reliable single-message communication. Using simulation, we evaluate various approaches in terms of correctness (with respect to data loss and duplication) and robustness to bad network conditions (high message loss rate and variable channel delays). Our results show that the SS approach is more robust, and has lower message overhead and higher goodput. Thus, SS presents the best choice for reliable applications, especially those operating over bandwidth-constrained, error-prone networks. This result also suggests that within a clean-slate transport architecture, explicit connection messages for data reliability are not needed, and so a simple common packet interface based on Delta-t—rather than TCP vs. T/TCP vs. UDP, etc. — can be provided to support both transactional and bulk, reliable and unreliable (unacknowledged) applications. I.

We revisit the problem of connection management for reliable transport. At one extreme, a pure soft-state (SS) approach (as in Delta-t [9]) safely removes the state of a connection at the sender and receiver once the state timers expire without the need for explicit removal messages. And new connections are established without an explicit handshaking phase. On the other hand, a hybrid hard-state/soft-state (HS+SS) approach (as in TCP) uses both explicit handshaking as well as timer-based management of the connection’s state. In this paper, we consider the worst-case scenario of reliable single-message communication, and develop a common analytical model that can be instantiated to capture either the SS approach or the HS+SS approach. We compare the two approaches in terms of goodput, message and state overhead. We also use simulations to compare against other approaches, and evaluate them in terms of correctness (with respect to data loss and duplication) and robustness to bad network conditions (high message loss rate and variable channel delays). Our results show that the SS approach is more robust, and has lower message overhead. On the other hand, SS requires more memory to keep connection states, which reduces goodput. Given memories are getting bigger and cheaper, SS presents the best choice over bandwidth-constrained, error-prone networks. I.

To my wife Esther...