4.1 Understanding the RTPS Message Header

The overall structure of an RTPS Message consists of a fixed-size RTPS Header followed by a variable number of RTPS Submessage parts. Each Submessage consists of a SubmessageHeader and a variable number of SubmessageElements. The RTPS header must appear at the beginning of every message.

The Header contains these fields:

• protocol: Identifies the message as an RTPS message.

Representation: 4 bytes

0.........8........16........24........32

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 

|   'R'   |   'T'   |   'P'   |   'S'   |

+---------+---------+---------+---------+

• version: Identifies the version of the RTPS protocol. .

Representation: 2 bytes. This release uses version {2, 1}.

• vendorId: Indicates the vendor that provides the implementation of the RTPS protocol.

Representation: 2 bytes. The RTI vendor identifier is {1, 1}.

• guidPrefix: Defines a default prefix to use for all GUIDs that appear in the message.

Representation: 12 bytes: 4 bytes for the host identifier, 4 bytes for the application identifier and 4 bytes for the instance identifier.

Figure 4.2: RTPS Message Header Structure

The LBRTPS transport reduces the RTPS message headers by eliminating the protocol, version, and vendorId fields in the RTPS Header structure.