Phillip T. Conrad

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.

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. >

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.

Animal tlatoque: attracting middle school students to computing through culturally-relevant themes

A popular approach to introducing students to computer science is to involve middle-school students in engaging programming activities. One challenge in such a program is attracting students who are not already positively predisposed to computing. In order to attract a diverse audience, we developed a summer program based on culturally-relevant themes that appealed to our two target audiences, females and Latina/os. This paper describes our success in developing and implementing a computing curriculum and recruiting materials for a 2-week summer camp integrating two themes, animal conservation and Mayan culture. Scratch programming was used to engage students in creating animations about animals and Mayan culture, allowing them an interdisciplinary experience that combined programming, culture, biology, art, and storytelling. Our recruiting efforts resulted in an application pool that was 73% female and 67% Latina/o, with only 6.5% in neither group. We had 34 students complete the program. Pre- and post- surveys showed that the number of students citing computer science as their top choice for a career doubled and interest in computer science as a career more than tripled.

SCTP in battlefield networks

The Stream Control Transmission Protocol (SCTP) is a new Internet standards track transport layer protocol SCTP was originally designed to transport PSTN signaling messages over IP networks, but is also capable of serving as a general purpose transport protocol. As such, SCTP provides an alternative that may be better able to satisfy the requirements of future battlefield networks than the traditional transport protocols, TCP and UDP. Unlike traditional transport protocols, SCTP allows multiple streams of messages within a single connection (or, in SCTP terminology, a single association). As the results in this paper show, this ability is particularly helpful in reducing latency for streaming multimedia in high loss environments. SCTP also provides features for multi-homing that may be helpful in high-mobility environments and additional security against denial-of-service attacks based on SYN flooding.

Partially reliable transport service

An analytic model is presented for a partially reliable transport protocol based on retransmissions. The model illustrates tradeoffs between two QoS parameters (delay and throughput), and various levels of reliability. The model predicts that the use of a reliable transport service when an application only needs a partially reliable one causes considerable throughput decreases and delay increases in lossy networks. On the other hand, over lossy networks, an unreliable transport service is unable to respect an applications loss tolerance. In lossy environments, partially reliable transport service avoids the extra cost of a reliable transport service, and, simultaneously, guarantees the minimal reliability that an application requires. A retransmission-based partially reliable transport service can be provided through either sender-based or receiver-based loss detection and recovery. Results show that both techniques provide almost identical reliability and delay. However, a sender-based approach provides better throughput than a receiver-based approach at high acknowledge loss rates.

Multimedia document retrieval system using partially ordered/partially reliable transport service

We investigate the benefits of using a partially-ordered/partially-reliable (PO/PR) transport service for multimedia document retrieval over the Internet by implementing a prototype system. We introduce PMSL, a document specification language combining aspects of Little and Ghafoors OCPN with features for graceful degradation. We then show that for retrieval of PMSL documents, our PO/PR transport protocol (POCv2) provides several benefits over traditional protocols such as TCP or UDP. First, POCv2 provides mechanisms to achieve the reliability and order requirements of individual multimedia objects as specified by a document author. Second, when network conditions are poor (e.g., high loss rates), POCv2 provides for graceful degradation. Finally, POCv2 simplifies application development by providing appropriate mechanisms for synchronization.

Network-conscious GIF image transmission over the Internet

Traditional image compression techniques seek the smallest possible file size for a given level of image quality. By contrast, network-conscious image compression techniques take into account the fact that a compressed image will be transmitted over a packet-switched network that may lose and reorder packets. We describe GIFNCa, a network-conscious revision of the popular GIF89a standard. As with GIF89a, GIFNCa compresses an image using LZW encoding, however GIFNCa does so using an Application Level Framing approach. The data is segmented into path MTU-size data units, each of which can be independently decompressed and displayed on its own. Under lossy network conditions, when used in combination with an unordered transport protocol, GIFNCa permits faster progressive display at the receiver than GIF89a over an ordered transport protocol. This advantage comes in exchange for a small penalty in overall compression. This paper defines GIFNCa, and presents preliminary experimental data concerning this tradeoff. The overall goal of this research is to illustrate (1) the value of considering network characteristics in designing image formats, and (2) the value of unordered transport service.

Testing environment for innovative transport protocols

This paper describes the development of a test environment for innovative transport protocols. Central to this work is the development of a universal transport library (UTL). The UTL is a library of transport protocols that provides application programmers the ability to write to a single application programming interface (API), then test their application with many different transport protocols. The UTL also allows for rapid prototyping of transport protocols at the user level. The UTL has been incorporated into two multimedia communication systems designed to provide better performance over lossy networks by using innovative transport protocol features: NETCICATS (a Network-Conscious Image Compression and Transmission System) and ReMDoR (a Remote Multimedia Document Retrieval system). These three tools facilitate the evaluation of flexible transport protocols and compression techniques for multimedia communications over lossy battlefield networks.

Network-Conscious Compressed Images over Wireless Networks

We apply the concept of network-consciousness to image compression, an approach that does not simply optimize compression, but which optimizes overall performance when compressed images are transmitted over a lossy packet-switched network such as the Internet. Using an Application Level Framing philosophy, an image is compressed into path MTU-size Application Data Units (ADUs) at the application layer. Each ADU carries its semantics, that is, it contains enough information to be processed independently of all other ADUs. Therefore each ADU can be delivered to the receiving application out-of-order, thereby enabling faster progressive display of images. We explain why this approach is useful in general and specifically for wireless/heterogeneous environments.