[DDS] ddsi-rtps specification

DDS DDSI-RTPS standard

background

• DDSI-RTPS： Full name “Real-time Publish Subscribe Protocol DDS Interoperability Wire Protocol”, It is DDS Wire-protocol. yes DDS Implement interoperability （ Standardization ） agreement .( It will be DDSI-RTPS, Referred to as RTPS)
• About DDS, Reference resources :
  【DDS】DDS And OpenDDS
• there Wire Protocol refer to ？ He is referring to a transmission mechanism .

A wire protocol is the mechanism for transmitting data from point a to point b.

• This article focuses on ：RTPS Specification , The structure of data interaction （ It can be roughly understood as network topology ）
• RTPS And the specific transmission mode used ( Such as UDP/IP) irrelevant , It defines a set for DDS The design of the “wire protocol”. It defines a set of PIM Model .DDS According to this model , Carry out specific implementation .PIM Modules include ：

1. Structure Module：RTPS How entities represent
2. Message Module：RTPS The message used indicates
3. Behavior Module： How to interact with messages , What messages are exchanged
4. Discovery Module： How to find between endpoints
   

• How to ensure reliable transmission ？

1. Reliability does not depend on specific implementation (TCP or UDP)
2. RTPS The reliability specification is defined

Data interaction

• I'm just going to focus on DDS(DDSI-RTPS) Network topology of . That is, the part of data interaction .
• DDS The interaction of entity data can be simply summarized into two parts ：

1. Discovery between endpoints ： Include Participain Discover And Endpoint Discovery
2. Between the ends ( Matched ) To transmit data : RTPS Writer And RTPS Reader The data transfer

Discover

• Discover Include :Participaint Discover and EndPoint(Writer/Reader) Discover

Participaint Discover

• Every Participant Contains built-in ParticipiantDataWriter And ParticipiantDataReader, as well as PariticipaintData(Topic), according to DDSI-RTPS The provisions of the ,Simple Participant Discovery Protocol（ all DDS What the implementer must achieve Pariticipanit Discovery standard ） Use the built-in Writer/Reade/Topic Realization Discovery.
• How to achieve Participaint Discovery？Participant Use ParticipiantDataWriter towards LocatorList( predefined ) Send a message to the address in ( regular ). Tell yourself that you are Active state .
• LocatorList：Unicast( Specify or automatically configure )+Multicast ( Specified by the concrete implementation )
  

EndPoint Discovery

• Participaint Discover After the Endpoint Discovery, In exchange for Endpoint Information
• Every Endpoint Contains built-in PubWriter/Reader、SubWriter/Reader And the corresponding built-in theme (topic). according to DDSI-RTPS The provisions of the , Simple Endpoint Discovery Protocol（ all DDS What the implementer must achieve Pariticipanit Discovery standard ） Use the built-in Writer/Reade/Topic Realization Discovery.
• EndPoint towards LocatorList(unicast and multicast) Send a message

Transport

• after Discovery after ,Writer And Reader Match each other . It can transmit data ( Subscribe to the Topic).
• The general process of data transmission is ：

1. DataWriter add to CacheChange To HistoryCache in （ Local cache ）
2. DataWriter take CacheChange Send it to the appropriate DataReader Of HistoryChace（ Local cache ）
3. HistoryCache（Reader） notice Reader There are new ones CacheChange.
4. Reader Reading data .
   

RTPS Two implementation reference models of the proposed entity :

Stateless Reference Implementation

• characteristic ：

no state on remote entities
improved scalability
less memory
more bandwidht usage

Stateful Reference Implementation

• characteristic ：

maintains full state on remote entities
reduced scalability
more memory
minimizes bandwidth usage

Depending on how much state is maintained, implementations may be a combination of the reference implementations.

Global data space

• About DDS The global data space is understood as follows ：

1. By existing in different physical locations HistoryCache Constructed global data space
2. Logically unified , Physically distributed
   

Reference material

• DDSI-RTPS v2.2

DDS series

【DDS】DDS And OpenDDS
【DDS】DDS-RPC Communication mechanism
【DDS】 be based on OpenDDS Of DDS-RPC Realization