This paper is about the mechanical verification of UDP based network programs. It uses the UDP portion of a formal model of the internetprotocols TCP (Transmission Control protocol) and UDP (userdatagramprotocol). ...
详细信息
ISBN:
(纸本)1920682236
This paper is about the mechanical verification of UDP based network programs. It uses the UDP portion of a formal model of the internetprotocols TCP (Transmission Control protocol) and UDP (userdatagramprotocol). The model includes asynchronous message passing, message loss and host failure. The model is based around the sockets library, the primary API used for writing UDP and TCP based applications. This paper demonstrates that formal, machine-checked, proof is possible in the UDP model by presenting the proof of a safety property for an implementation of Stenning's protocol. The protocol is implemented in a fragment of the OCaml language, using the sockets library for UDP network communication. The entire development including the safety proof is carried out in the proof assistant Isabelle; this assures soundness. Thus this paper demonstrates that it is possible to machine verify very concrete representations of distributed programs in a detailed semantics that accurately reflects the programs execution environment. Previously only abstract representations of this protocol have been machine verified. The proof, based on an implementation, provides a contrast to other verifications.
In the last few years an increasing amount of attention has been paid to technologies for the transmission of voice over IP (VoIP). At present, the UDP transport protocol is used to provide this service. However, when...
详细信息
In the last few years an increasing amount of attention has been paid to technologies for the transmission of voice over IP (VoIP). At present, the UDP transport protocol is used to provide this service. However, when the same bottleneck link is shared with TCP flows, and in the presence of a high network load and congestion, UDP sources capture most of the bandwidth, strongly penalizing TCP sources. To solve this problem some congestion control should be introduced for UDP traffic as well, in such a way that this traffic becomes TCP-friendly. In this perspective, several TCP-friendly algorithms have been proposed in the literature. Among them, the most promising candidates for the immediate future are RAP and TFRC. However, although these algorithms were introduced to support real-time applications on the internet, up to now the only target in optimizing them has been that of achieving fairness with TCP flows in the network. No attention has been paid to the applications using them, and in particular, to the quality of service (QoS) perceived by their users. The target of this paper is to analyze the problem of transmitting voice over IP when voice sources use one of these TCP-friendly algorithms. With this aim, a VoIP system architecture is introduced and the characteristics of each its elements are discussed. To optimize the system, a multirate voice encoder is used so as to be feasible to work over a TCP layer, and a modification of both RAP and TFRC is proposed. Finally, in order to analyze the performance of the proposed system architecture and to compare the modified RAP and TFRC with the original algorithms, the sources have been modeled with an arrival process modulated by a Markov chain, and the model has been used to generate traffic in a simulation study performed with the ns-2 network simulator. Copyright (C) 2003 AEI.
Active pyruvate Pi dikinase in leaf or chloroplast [from Zea mays cv. Dekalb 805A] extracts isolated from illuminated leaves was inactivated by incubating with ADP. With chloroplast extracts neither ATP nor AMP alone ...
详细信息
Active pyruvate Pi dikinase in leaf or chloroplast [from Zea mays cv. Dekalb 805A] extracts isolated from illuminated leaves was inactivated by incubating with ADP. With chloroplast extracts neither ATP nor AMP alone was effective. Half the maximum rate of inactivation was observed with about 55 .mu.M ADP. Apparently, ADP-mediated inactivation had a co-requirement for low concentrations of ATP [Buchanan];adding hexokinase and glucose prevented inactivation by ADP [Feldhaus et al.] and when GDP and UDP were added in place of ADP they mediated rapid inactivation only when ATP was also provided;GTP was not effective. ATP was apparently optimally effective at .apprx. 1 .mu.M or less. The rate of inactivation was approximately proportional to the square of extract concentration, suggesting dependency on a factor in the extracts in addition to active enzyme. The involvement of one or more heat-labile protein factors was confirmed by trypsin treatment of extracts. Pyruvate Pi dikinase inactivated by treatment with ADP was reactivated by incubating with Pi, a property common to the inactive enzyme extracted from darkened leaves. Thiol/disulfide interconversion was apparently not critical in the regulation of pyruvate Pi dikinase.
5-Mercaptouridine-5′-diphosphate (hs5UDP) has been synthesized and investigated as a substrate of the polynucleotide phosphorylase ofMicrococcusluteus. While hs5UDP is not utilized alone, It can be copolymerized with...
详细信息
5-Mercaptouridine-5′-diphosphate (hs5UDP) has been synthesized and investigated as a substrate of the polynucleotide phosphorylase ofMicrococcusluteus. While hs5UDP is not utilized alone, It can be copolymerized with UDP; however, unusually for this enzyme, the ratio of 5-mercaptouridylatevs. uridylate residues in the polynucleotlde product (MPU) is always lower than the ratio of hs5UDPvs. UDP in the substrate mixture. Furthermore, hs5UDP decreases the rate of the enzymic polymerization reaction. The MPU productforms two-stranded and three-stranded complexes with poly(A). The circular dichroic spectra of these complexes are similar to those formed between poly(U) and poly(A), but their melting profiles indicate somewhat lower stability. The physicochemical and biochemical properties of the enzymic product are qualitatively similar to those of MPU prepared by chemical modification; both are potent inhibitors of a DNA-dependent RNA polyrmeras
暂无评论