Irene Rüngeler

MPI-NeTSim: A Network Simulation Module for MPI

Programs that execute in parallel across a network often use the Message Passing Interface (MPI) library for communication. The network requirements of an MPI program are often unclear because of the difficulty in exploring alternative network configurations as well as obtaining packet level information about the communication. MPI-NeTSim is an execution environment to emulate MPI programs on simulated networks to allow users to better explore the impact of the network on the performance of MPI programs. We describe the design of MPI-NeTSim and the integration of OMNeT++’s INET framework into MPICH2’s MPI middleware. We introduce a novel technique for uniformly slowing down the execution of the system to allow the discrete event network simulator to keep up with the execution and provide a consistent view of the communication. We validate our technique with synthetic programs as well as the standard NAS benchmarks. We demonstrate MPI-NeTSim’s usefulness in analyzing the effect of the network on communication by using our environment to study the impact of a slow-link on the NAS benchmarks.

WebRTC Data Channels

Browser-to-browser real-time communication is making rapid progress in the standardization process, both in the Internet Engineering Task Force (IETF) and the World Wide Web Consortium (W3C). These advancements cover many aspects, such as interface definitions, protocol mechanisms and security. Many scenarios discussed show that interfaces and standards can serve as a foundation for interoperable real-time multimedia applications that are easy to implement. In addition to the transfer of real-time-media data, the currently discussed protocol stacks also support transfer of non-media data. This article gives a brief top-down discussion of how the standardization efforts reflect the use cases of non-media data transfer, which use cases could be addressed, and how technologies are applied to achieve the goals. Furthermore, it provides an insight into specific protocol mechanisms as well as unresolved issues for future work.

Portable and Performant Userspace SCTP Stack

One of only two new transport protocols introduced in the last 30 years is the Stream Control Transmission Protocol (SCTP). SCTP enables capabilities like additional throughput and fault tolerance for multihomed hosts. An SCTP implementation is included with the Linux kernel and another implementation called sctplib functions successfully in userspace on several platforms but unfortunately neither of these implementations have all of the latest features nor do they perform as well as the FreeBSD kernel implementation of SCTP. We were motivated to produce a portable implementation of the FreeBSD kernel SCTP stack that operates in userspace of any system because of both our desires to obtain a higher performance SCTP stack for Linux as well as to exploit recent developments in hardware virtualization and transport protocol onloading. Unlike any other userspace transport implementation for TCP or SCTP, our userspace SCTP stack simultaneously achieves similar throughput and latency as the Linux kernel TCP stack, without compromising on any of the transports features as well as maintaining true portability across multiple operating systems and devices. We create a callback API and implement a threshold to control its usage; our userspace SCTP stack with these optimizations obtains higher throughput than the Linux kernel implementation of SCTP. We describe our userspace SCTP stacks design and demonstrate how it gives similar throughput and latency on Linux as the kernel TCP implementation, with the benefits of the new features of SCTP.

Network Address Translation for the Stream Control Transmission Protocol

Network address translation is widely deployed in the Internet and supports the transmission control protocol and the user datagram protocol as transport layer protocols. Although part of the kernels of all recent Linux distributions, namely, the FreeBSD 7 and the Solaris 10 operating systems, the new Internet Engineering Task Force transport protocol - stream control transmission protocol - is not supported on most NAT middleboxes yet. This article discusses the deficiencies of using existing NAT methods for SCTP and describes a new SCTP-specific NAT concept. This concept is analyzed in detail for several important network scenarios, including peer-to-peer, transport layer mobility, and multihoming.

Improving the Acknowledgment Handling of SCTP

In general, the Stream Control Transmission Protocol (SCTP) delays the sending of acknowledgments. For specific cases, this results in non-optimal protocol behavior. Therefore, the protocol specification defines several exceptions that allow the receiver of an SCTP packet containing user data to send the corresponding acknowledgment without delaying it. However, all these exceptions only take the status at the user data receiver side into account and thus cannot cover all relevant situations. To overcome this limitation, the authors have proposed a generic mechanism allowing the sender of an SCTP packet containing user data to indicate, that the corresponding acknowledgment should not be delayed. This paper analyzes the benefits of using this sender side mechanism under several conditions.