Real-Time Publish-Subscribe (RTPS)


RTPS protocol was developed by Real-Time Innovations, Inc. as wire protocol for Data Distribution System.

This page contains only a short introduction to RTPS. For more detailed information, you can access the following sources:

Introduction

With the explosion of the Internet, the TCP/UDP/IP protocol suite has become the underlying framework upon which all Internet-based communications are built. Their success attests to the generality and power of these protocols. However, these transport-level protocols are too low level to be used directly by any but the simplest applications. Consequently, higher-level protocols such as HTTP, FTP, DHCP, DCE, RTP, DCOM, and CORBA have emerged. Each of these protocols fills a niche, providing well-tuned functionality for specific purposes or application domains.

In network communications, as in many fields of engineering, it is a fact that one size does not fit all. Engineering design is about making the right set of trade-offs, and these trade-offs must balance conflicting requirements such as generality, ease of use, richness of features, performance, memory size and usage, scalability, determinism, and robustness. These trade-offs must be made in light of the types of information flow (e.g. periodic, one-to-many, request-reply, events), and the constraints imposed by the application and execution platforms.

The Real-Time Publish-Subscribe (RTPS) Wire Protocol provides two main communication models: the publish-subscribe protocol, which transfers data from publishers to subscribers; and the Composite State Transfer (CST) protocol, which transfers state.

The RTPS protocol is designed to run over an unreliable transport such as UDP/IP. The broad goals for the RTPS protocol design are:

The RTPS Protocol runs in a Domain of DomainParticipants. A DomainParticipant contains local CommunicationEndpoints through which it sends or receives information using the RTPS Protocols. The CommunicationEndpoints are either Readers or Writers. Writers provide locally available data (a composite state or a stream of issues) on the Domain. Readers obtain this information.

There are two broad classes of Writers: Publications and CSTWriters. A Publication is a Writer that provides issues to one or more instances of a Subscription using the publish-subscribe protocol and semantics.

The presence of a Publication in an DomainParticipant indicates that the DomainParticipant is willing to publish issues to matching subscriptions on the Domain. The attributes of the Publication describe the contents (the topic), the type of the issues, and the quality of the stream of issues that is published on the Domain.

There are two broad classes of Readers: Subscriptions and CSTReaders. A Subscription is a Reader that receives issues from one or more instances of Publication, using the publish-subscribe protocol.

The presence of a Subscription indicates that the DomainParticipant wants to receive issues from Publications for a specific topic in the Domain. The Subscription has attributes that identify the contents (the topic) of the data, the type of the issues and the quality with which it wants to receive the stream of issues.

The CSTWriter and CSTReader are the equivalent of the Publication and Subscription, respectively, but are used as communication end-points of the state-synchronization protocol (CST).

Every Reader (CSTReader or Subscription) and Writer (CSTWriter or Publication) is part of an DomainParticipant. The DomainParticipant and its Readers and Writers are local, which is indicated in Figure 1.1 by the keyword "local" on the relationship between an DomainParticipant and its CommunicationEndpoints.

There are two kinds of DomainParticipants: Managers and ManagedApplications. A Manager is a special DomainParticipant that helps ManagedApplications automatically discover each other within the Domain. A ManagedApplication is an DomainParticipant that is managed by one or more Managers. Every ManagedApplication is managed by at least one Manager.

The protocol provides two types of functionality:

The Basic RTPS Transport Interface

RTPS is designed to run on an unreliable transport mechanism, such as UDP/IP. The protocols implement reliability in the transfer of issues and state.

RTPS takes advantage of the multicast capabilities of the transport mechanism, where one message from a sender can reach multiple receivers.

RTPS is designed to promote determinism of the underlying communication mechanism. The protocol also provides an open trade-off between determinism and reliability.

The Basic Logical Messages

The RTPS protocol uses five logical messages:

Each of these logical messages are sent between specific Readers and Writers as follows:

Readers and Writers are both senders and receivers of RTPS Messages. In the protocol, the logical messages ISSUE, VAR, HEARTBEAT, GAP and ACK can be combined into a single message in several ways to make efficient use of the underlying communication mechanism. Chapter 3 explains the format and construction of a Message.

Underlying Wire Representation

RTPS uses the CDR (Common Data Representation) as defined by the Object Management Group (OMG) to represent all basic data and structures.

Protocols/rtps (last edited 2011-09-02 18:27:11 by BillMeier)