Daniele Lacamera

Analysis of TCP and DTN Retransmission Algorithms in Presence of Channel Disruptions

“Challenged Networks” often violate the TCP design key assumption of continuous path availability from the source to the sink node. These networks are the preferred target of Delay/Disruption Tolerant Networking (DTN), which aims at providing a more robust network architecture. Against disruptions, however, even standard TCP offers a certain level of robustness, through its many retransmission algorithms. The aim of the paper is to present a comparative analysis of the TCP and DTN Bundle protocol retransmission algorithms triggered by channel disruptions. This analysis provides the reader with a comprehensive view of the many mechanisms involved and a first assessment of their impact on some performance metrics. Although DTN put on top of TCP clearly offers higher resilience against long disruptions, TCP itself can provide a certain level of robustness, provided that the many retransmission parameters are properly set. The analysis is completed by some numerical evaluations on a GEO satellite channel obtained by means of a Linux testbed.

Analysis of TCP live experiments on a real GEO satellite testbed

The paper describes a measurement campaign carried out by the University of Bologna (UoB), the National Inter-University Consortium for Telecommunications (CNIT) and the University of California Los Angeles (UCLA). The aim of the experiments was the performance assessment of a wide range of TCP enhancements on network environments that include a real GEO satellite link. To this end, UoB and CNIT integrated their network tools and set up a testbed composed of a cluster of UoB Linux PCs connected to the CNIT GEO Skyplex satellite platform. Tests were conducted considering both end-to-end TCP enhancements and a TCP-splitting Performance Enhancing Proxy (PEP) developed by UoB. The analysis was not limited to isolated satellite links, but embraced more complex heterogeneous networks, where satellite connections have to compete with wired cross traffic for the network resources. The analysis of the large set of experimental data presented in the paper confirms the challenges posed by GEO satellite channels, which, in this case, were worsened also by the presence of Bandwidth on Demand technique, which impacts TCP performance. Among the end-to-end TCP variants, best results are generally shown by TCP Hybla, especially when heterogeneous environments are considered. On the other hand, the splitting PEP solution offers the best performance with respect to all TCP enhancements, due to its ability to isolate satellite channel impairments. The possible drawback of this approach is in that it violates the end-to-end semantics and is incompatible with IPsec protocol implementation.

TCP, PEP and DTN performance on disruptive satellite channels

Fixed GEO satellite communications are impaired by long RTTs and the possible presence of packet losses on the satellite radio channel. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions can cause further performance deterioration. In this paper, we present a preliminary assessment of disruption impact on performance comparing different approaches, such as end-to-end TCP, PEP based on TCP splitting and DTN. Performance is evaluated on a Linux testbed, by emulating the satellite link interruptions caused by tunnels on a real railway line. Results show that good performance can be achieved by all of the approaches considered provided that on the satellite channel an efficient TCP variant, like Hybla, is adopted; however, DTN offers best performance. DTN superiority emerges when the disrupted satellite channel is the last leg of a congested path, thanks to the DTN custody transfer mechanism and store and forward capability.

The TATPA Testbed; A Testbed for Advanced Transport Protocols and Architecture performance evaluation on wireless channels

The paper describes the Testbed for Advanced Transport Protocols and Architecture (TATPA) developed by the University of Bologna, inside the SatNEx (Satellite Network of Excellence) framework. The aim is to provide partners, and in general the scientific community, with a powerful and flexible emulation tool to evaluate new proposal at transport layer (both protocols and architectures). In accordance with our research interests, the testbed is focused on performance evaluation on heterogeneous networks that include satellite links. The testbed architecture is based on a cluster of Linux PCs and, in addition to some widely adopted networking tools (NistNet, Ipcrf), it also exploits software packages specifically developed by the authors (Multi TCP, PEPsal, released under GNU license). The testbed is controlled by remote through a web interface, which provides both users and administrators with a ubiquitous easy access to the testbed features. A dedicated PC, acting as testbed controller, hosts the web server and the control engine, developed in PHP and bash and based on a MySQL database. In the paper, a particular attention is devoted to the description of the innovative design and the many advanced features of this controller. An example of use, seen from the final user point of view, completes the description of the TATPA testbed.

An integrated testbed for wireless advanced transport protocols and architectures

The challenges posed by data communications over both satellite and terrestrial wireless systems make the adoption of adequate countermeasures advisable. In particular, the poor performance experienced by TCP transport protocol in such environments has fed the scientific community to propose novel solutions, from both the protocols and architecture points of view. In order to perform a thorough assessment of such proposals, University of Bologna (UoB) and National Inter-University Consortium for Telecommunications (CNIT) have conceived and set-up an integrated testbed. Its characteristics and capabilities are presented and discussed in this work, focusing in particular on the advantages offered by the ubiquitous remote control of the testbed, provided by a powerful web interface specifically designed for this purpose

DTN meets smartphones: Future prospects and tests

Although first conceived for the Interplanetary Internet, DTN (Delay-/Disruption-Tolerant Networking) architecture has successively enlarged its application scope to embrace all “challenged networks”, i.e. networks where the TCP design key assumptions of short RTT, no disruptions, and continuous end-to-end path availability are challenged. The distinctive features of DTN architecture seem ideal for the connectivity problems of modern smartphones. These are both PCs and cell phones and are characterized by a wide range of different communications technologies, including Bluetooth, WiFi, and advanced 3G radio interfaces, like UMTS HSPA (High-Speed Packet Access). The aim of this paper is to show that it is both feasible and useful to adopt DTN architecture on these advanced devices, in addition to the usual TCP/IP protocol stack. To this end, a series of tests is presented and discussed in the paper, considering various communication interfaces, 3G in particular. Tests were greatly facilitated by the introduction onto the market of some smartphones running Linux based operating systems, like Maemo, which allowed the authors the direct use of “official” DTN code (i.e. the DTN2 reference implementation and the DTNperf_2 evaluation tool).

TCP Live Experiments on a Real GEO Satellite Testbed

The paper describes a measurement campaign recently carried out by University of Bologna (UoB), National Inter-University Consortium for Telecommunications-(CNIT) and University of California Los Angeles (UCLA), on the CNIT GEO Skyplex platform. The aim of the experiments is the performance assessment of a wide range of TCP enhancements on network environments that include a real GEO satellite link. Measurements were carried out by means of the integrated UoB-CNIT testbed, fully controlled by remote through a Web interface. Analysis of live experiments data confirm the challenges posed by GEO sat channels, and show a negative influence of the Skyplex BoD technique on TCP start-up performance. Among the TCP enhancements compared in the paper, best results are provided by TCP Hybla and TCP Westwood, with an advantage of the former in heterogeneous environments characterized by a high level of RTT unfairness.

Virtualization Technologies for DTN Testbeds

At present, Internet is based on the availability of a continuous path from the source to the sink node and on limited delays. These assumptions do not hold in “challenged networks”, which comprise a wide variety of different environments, from sensor networks to space communications (including satellite systems). These networks are the preferred target of Delay/Disruption Tolerant Networking (DTN), an innovative networking architecture able to cope with long delays, channel disruptions and limited or intermittent connectivity. Given the increasing interest in DTN, there is urgent need for suitable tools for DTN performance evaluation. In general, there are two approaches to performance evaluation in networking: simulation and real testbeds. In this paper, after an in-depth discussion of advantages and disadvantages of both, a third way based on a virtualization is proposed and tested for DTN environments, for which it seems particularly suitable. To validate this assumption, a virtual counterpart of a real testbed is set-up using Virtual Testbed Toolkit (VTT) components. A series of tests is then performed by considering DTN transmission on a heterogeneous network including a GEO satellite link. The close match between real and virtual testbed results confirms the validity of the virtual approach for accurate performance evaluations in DTN environments.

Virtualbricks for DTN Satellite Communications Research and Education

Virtualbricks is a virtualization solution for GNU/Linux platforms developed by the authors and included in Debian. The paper aims to show its potential, referring to version 1.0, just released, when applied to both research and education on DTN satellite communications. In brief, Virtualbricks is a frontend for the management of Qemu/KVM Virtual Machines (VMs) and VDE virtualized network devices (switches, channel emulators, etc.). It can be used to manage either isolated VMs, or testbeds consisting of many VMs interconnected by VDE elements. Among the wide variety of possible applications, with or without VM interconnections, the focus here is on the development of a virtual testbed on DTN satellite communications, a task for which Virtualbricks was especially designed. After having introduced the main characteristics of Virtualbricks, in the paper we will show how to set-up a Virtualbricks testbed, taking as an example a testbed recently used by the authors to investigate Moon communications through orbiters. The validity of Virtualbricks results is confirmed by comparison with results achieved on a real testbed, set-up for this purpose. The same testbed has also been successfully used for educational purposes at the University of Bologna.

PEPsal Performance Analysis on Disruptive Radio Channels

Fixed GEO satellite communications are impaired by long RTTs (especially GEO) and the possible presence of packet losses on the satellite radio channels. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions associated with the presence of shadowing can cause further performance deterioration. In this paper, we evaluate the impact of a disruptive channel on PEPsal, a TCP-splitting PEP previously developed by the authors. Results, obtained by emulating the satellite link interruptions caused by tunnels of a real railway line, highlight the advantages of the TCP-splitting architecture. By enabling the adoption of optimized version of TCP on the satellite connection and a satellite-specific tuning of TCP parameters, PEPsal can offer a significant resilience against all kind of satellite impairments.