Markku Kojo

Bio: Markku Kojo is an academic researcher from University of Helsinki. The author has contributed to research in topics: TCP acceleration & TCP global synchronization. The author has an hindex of 21, co-authored 59 publications receiving 2411 citations.

Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations

Abstract: This document is a survey of Performance Enhancing Proxies (PEPs) often employed to improve degraded TCP performance caused by characteristics of specific link environments, for example, in satellite, wireless WAN, and wireless LAN environments. Different types of Performance Enhancing Proxies are described as well as the mechanisms used to improve performance. Emphasis is put on proxies operating with TCP. In addition, motivations for their development and use are described along with some of the consequences of using them, especially in the context of the Internet.

Mobility Related Terminology

Abstract: There is a need for common definitions of terminology in the work to be done around IP mobility. This document defines terms for mobility related terminology. The document originated out of work done in the Seamoby Working Group but has broader applicability for terminology used in IETF-wide discourse on technology for mobility and IP networks. Other working groups dealing with mobility may want to take advantage of this terminology. This memo provides information for the Internet community.

F-RTO: an enhanced recovery algorithm for TCP retransmission timeouts

Abstract: Spurious TCP retransmission timeouts (RTOs) have been reported to be a problem on network paths involving links that are prone to sudden delays due to various reasons. Especially many wireless network technologies contain such links. Spurious retransmission timeouts often cause unnecessary retransmission of several segments, which is harmful for TCP performance. Recent proposals for avoiding unnecessary retransmissions after a spurious RTO require use of TCP options which must be implemented and enabled at both ends of teh connection. We introduce a new TCP sender algorithm for recovery after a retransmission timeout and show that unnecessary retransmissions after a spurious retransmission timeout, improving the TCP performance considerably. The algorithm is friendly towards other TCP connections, because it follows the congestion control principles and injects packets to the network at same rate as a conventional TCP sender. We implemented the algorithm and compared its performance to conventional TCP and Eifel TCP when RTOs occurred either due to sudden delays or due to packet losses. The results show that our algorithm either improves performance or gives similar througput as the other TCP variants evaluated in different test cases.

Optimizing World-Wide Web for weakly connected mobile workstations: An indirect approach

Abstract: Modern portable computers and wireless connections over cellular telephone networks have created a new platform for distributed information processing. We have designed a communication architecture that makes it possible to exploit the existing TCP/IP communication protocols but that also takes into account specific features of cellular links. Our communication architecture is based on the concept of indirect interaction. The mediating interceptor is the bridge between the worlds of wireless and wireline communication. It also provides enhanced functionality that improves fault-tolerance and performance. In this paper we demonstrate how the architecture is used to improve the performance of the WWW information browsing. Similar solution methods can be applied to other existing applications and protocols. >

Long Thin Networks

Abstract: In view of the unpredictable and problematic nature of long thin networks (for example, wireless WANs), arriving at an optimized transport is a daunting task. We have reviewed the existing proposals along with future research items. Based on this overview, we also recommend mechanisms for implementation in long thin networks.

Ad hoc On-Demand Distance Vector (AODV) Routing

Abstract: The Ad hoc On-Demand Distance Vector (AODV) routing protocol is intended for use by mobile nodes in an ad hoc network. It offers quick adaptation to dynamic link conditions, low processing and memory overhead, low network utilization, and determines unicast routes to destinations within the ad hoc network. It uses destination sequence numbers to ensure loop freedom at all times (even in the face of anomalous delivery of routing control messages), avoiding problems (such as "counting to infinity") associated with classical distance vector protocols.

A delay-tolerant network architecture for challenged internets

Abstract: The highly successful architecture and protocols of today's Internet may operate poorly in environments characterized by very long delay paths and frequent network partitions. These problems are exacerbated by end nodes with limited power or memory resources. Often deployed in mobile and extreme environments lacking continuous connectivity, many such networks have their own specialized protocols, and do not utilize IP. To achieve interoperability between them, we propose a network architecture and application interface structured around optionally-reliable asynchronous message forwarding, with limited expectations of end-to-end connectivity and node resources. The architecture operates as an overlay above the transport layers of the networks it interconnects, and provides key services such as in-network data storage and retransmission, interoperable naming, authenticated forwarding and a coarse-grained class of service.

I-TCP: indirect TCP for mobile hosts

Abstract: IP based solutions to accommodate mobile hosts within existing internetworks do not address the distinctive features of wireless mobile computing. IP-based transport protocols thus suffer from poor performance when a mobile host communicates with a host on the fixed network. This is caused by frequent disruptions in network layer connectivity due to i) mobility and ii) unreliable nature of the wireless link. We describe I-TCP, which is an indirect transport layer protocol for mobile hosts. I-TCP utilizes the resources of Mobility Support Routers (MSRs) to provide transport layer communication between mobile hosts and hosts on the fixed network. With I-TCP, the problems related to mobility and unreliability of wireless link are handled entirely within the wireless link; the TCP/IP software on the fixed hosts is not modified. Using I-TCP on our testbed, the throughput between a fixed host and a mobile host improved substantially in comparison to regular TCP.

Congestion control for high bandwidth-delay product networks

Abstract: Theory and experiments show that as the per-flow product of bandwidth and latency increases, TCP becomes inefficient and prone to instability, regardless of the queuing scheme. This failing becomes increasingly important as the Internet evolves to incorporate very high-bandwidth optical links and more large-delay satellite links.To address this problem, we develop a novel approach to Internet congestion control that outperforms TCP in conventional environments, and remains efficient, fair, scalable, and stable as the bandwidth-delay product increases. This new eXplicit Control Protocol, XCP, generalizes the Explicit Congestion Notification proposal (ECN). In addition, XCP introduces the new concept of decoupling utilization control from fairness control. This allows a more flexible and analytically tractable protocol design and opens new avenues for service differentiation.Using a control theory framework, we model XCP and demonstrate it is stable and efficient regardless of the link capacity, the round trip delay, and the number of sources. Extensive packet-level simulations show that XCP outperforms TCP in both conventional and high bandwidth-delay environments. Further, XCP achieves fair bandwidth allocation, high utilization, small standing queue size, and near-zero packet drops, with both steady and highly varying traffic. Additionally, the new protocol does not maintain any per-flow state in routers and requires few CPU cycles per packet, which makes it implementable in high-speed routers.