Paul D. Amer

Concurrent multipath transfer using SCTP multihoming over independent end-to-end paths

Concurrent multipath transfer (CMT) uses the Stream Control Transmission Protocols (SCTP) multihoming feature to distribute data across multiple end-to-end paths in a multihomed SCTP association. We identify three negative side-effects of reordering introduced by CMT that must be managed before efficient parallel transfer can be achieved: (1) unnecessary fast retransmissions by a sender; (2) overly conservative congestion window (cwnd) growth at a sender; and (3) increased ack traffic due to fewer delayed acks by a receiver. We propose three algorithms which augment and/or modify current SCTP to counter these side-effects. Presented with several choices as to where a sender should direct retransmissions of lost data, we propose five retransmission policies for CMT. We demonstrate spurious retransmissions in CMT with all five policies and propose changes to CMT to allow the different policies. CMT is evaluated against AppStripe, which is an idealized application that stripes data over multiple paths using multiple SCTP associations. The different CMT retransmission policies are then evaluated with varied constrained receive buffer sizes. In this foundation work, we operate under the strong assumption that the bottleneck queues on the end-to-end paths used in CMT are independent.

The transport layer: tutorial and survey

Transport layer protocols provide for end-to-end communication between two or more hosts. This paper presents a tutorial on transport layer concepts and terminology, and a survey of transport layer services and protocols. The transport layer protocol TCP is used as a reference point, and compared and contrasted with nineteen other protocols designed over the past two decades. The service and protocol features of twelve of the most important protocols are summarized in both text and tables.

SCTP: a proposed standard for robust Internet data transport

The stream control transmission protocol (SCTP) is an evolving general purpose Internet transport protocol designed to bridge the gap between TCP and UDP. SCTP evolved from a telephony signaling protocol for IP networks and is now a proposed standard with the Internet Engineering Task Force. Like TCP, SCTP provides a reliable, full-duplex connection and mechanisms to control network congestion. However, SCTP expands transport layer possibilities beyond TCP and UDP, offering new delivery options that are particularly desirable for telephony signaling and multimedia applications.

Partial-order transport service for multimedia and other applications

Investigates a partial-order connection (POC) service/protocol. Unlike classic transport services that deliver objects either in the exact order transmitted or according to no particular order, POC provides a partial-order service, i.e. a service that requires some, but not all objects to be received in the order transmitted. Two versions of POC are proposed: reliable, which requires that all transmitted objects are eventually delivered, and unreliable, which permits the service to lose a subset of the objects. In the unreliable version, objects are more finely categorized into one of three reliability classes depending on their temporal value. Two metrics based on e/sub i/(P), the number of linear extensions of partial-order P in the presence of i lost objects, are proposed as complexity measures of different combinations of partial order and reliability. Formulae for calculating e/sub i/(P) are derived when P is series-parallel. A formal specification of a POC protocol, written in Estelle, is presented and discussed. This specification was designed and validated using formal description tools and provides a basis for future implementations. >

Performance implications of a bounded receive buffer in concurrent multipath transfer

We study the performance of Concurrent Multipath Transfer using SCTP multihoming (CMT) in the presence of a bounded receive buffer (rbuf). We demonstrate using simulation that if two paths are used for CMT, the lower quality (i.e., higher loss rate) path degrades overall throughput of an rbuf-constrained CMT association by blocking the rbuf. We argue that rbuf blocking is not specific to the transport layer, but applies to multipath transfers at other layers as well. We present and discuss CMT performance using several retransmission policies and various constrained rbuf values. We also study the impact of rbuf blocking with different combinations of end-to-end loss rate and delay on the two paths and show that when large differences exist in path delays and loss rates, using only the better path outperforms using two paths concurrently. While rbuf blocking cannot be eliminated, it can be reduced by choice of retransmission policy - a mechanism available to only the transport layer.

Retransmission policies for concurrent multipath transfer using SCTP multihoming

Concurrent multipath transfer (CMT) uses the stream control transmission protocols (SCTPs) multihoming feature to distribute data across multiple end-to-end paths in a multihomed SCTP association. We propose five retransmission policies for CMT. We demonstrate the occurrence of spurious retransmissions in CMT with all of the five policies, and propose an amendment to the timeout retransmission mechanism to avoid spurious retransmissions. We also modify the Cwnd Update for CMT (CUC) algorithm to allow better cwnd growth in CMT with the different retransmission policies. We then evaluate the retransmission policies using ns-2 simulations, and discuss the distributions of traffic that result. We operate under the strong assumptions that the receivers advertised window does not constrain the sender, and that the bottleneck queues on the end-to-end paths used in CMT are independent.

Receive buffer blocking in concurrent multipath transfer

Previously, we studied the performance of concurrent multipath transfer using SCTP multihoming (CMT) under the assumption of an infinite receive buffer (rbuf). Here, we study CMT performance when a sender is constrained by the rbuf. We demonstrate using simulation that if two paths are used for CMT, the lower quality (i.e., higher loss rate) path degrades overall throughput of an rbuf-constrained CMT association by blocking the rbuf. We demonstrate that a wise retransmission policy can alleviate some of the throughput degradation by reducing the rbuf blocking problem. We present and discuss CMT performance using several retransmission policies and constrained rbuf values of 16 KB, 32 KB, 64 KB, 128 KB, and 256 KB. While rbuf blocking cannot be eliminated, it can be reduced by choice of retransmission policy - a facility available to only the transport layer.

Management of sampled real-time network measurements

An overview is presented of the statistical sampling of measurements from a computer network, a necessary task in all five facilities of the proposed ISO network management architecture. Network measurement, viewed from the standpoint of real-time status reporting, is performed either to approximate the condition of a network parameter over a given interval or to detect a significant change in system behavior over consecutive intervals. Appropriate statistical tools for sampling and analyzing traffic in real time are presented.<<ETX>>

Retransmission-based partially reliable transport service: an analytic model

Many applications such as video and audio can tolerate loss. When the network layer provides a best-effort service such as on the Internet, the loss rate of the underlying network service may be higher than an applications tolerance for loss. This paper analytically studies retransmission-based partially reliable transport (layer protocol) service. Results show that a partially reliable transport service provides increasingly higher throughput and lower delay than a reliable transport service as an applications loss tolerance increases and as the underlying network service gets more lossy. Also, to some degree, a partially reliable transport service eases the negative effects of ack losses on throughput. Three cost functions associated with the reliability level that a system can support are introduced. These cost functions help demonstrate the penalty when a transport service does not support the ideal reliability level for an application. Results show that the use of a reliable transport service when an application only needs a partially reliable transport service can cause considerable throughput drops and delay increases in lossy networks. On the other hand, at high loss rates, an unreliable transport service is unable to respect an applications loss tolerance. Thus, in lossy environments, a partially reliable transport service is necessary to avoid the extra cost of a reliable transport service, and, at the same time, to guarantee the minimal reliability that an application requires.

SCTP: an innovative transport layer protocol for the web

We propose using the Stream Control Transmission Protocol (SCTP), a recent IETF transport layer protocol, for reliable web transport. Although TCP has traditionally been used, we argue that SCTP better matches the needs of HTTP-based network applications. This position paper discusses SCTP features that address: (i) head-of-line blocking within a single TCP connection, (ii) vulnerability to network failures, and (iii) vulnerability to denial-of-service SYN attacks. We discuss our experience in modifying the Apache server and the Firefox browser to benefit from SCTP, and demonstrate our HTTP over SCTP design via simple experiments. We also discuss the benefits of using SCTP in other web domains through two example scenarios ? multiplexing user requests, and multiplexing resource access. Finally, we highlight several SCTP features that will be valuable to the design and implementation of current HTTP-based client-server applications.