The document outlines the DDS-XRCE (Data Distribution Service for Extremely Resource-Constrained Environments) specifications presented at an OMG meeting in June 2017, emphasizing its agent-based deployment model and interoperability for constrained networks. It details the architecture, message structure, and object model that supports efficient communication in low-bandwidth scenarios while maintaining compatibility with existing DDS systems. Additionally, it discusses security measures, transport requirements, and the potential for merging ongoing submissions from different contributors in the future.

1. DDS-XRCE
DDS for Extremely Resource Contained Networks
Dra< specifica?on presented at OMG Brussels, June 2017
mars/17-06-11
Joint Submission: RTI, TwinOaks, eProsima
Contact: Gerardo Pardo-Castellote, Ph.D.

3. Outline
• Background
• High Level Overview
• Key Concepts and Features
• An example
• Status
©2017 Real-Time Innova?ons, Inc.

4. Backgrond
• Two ini?al submissions in Chicago, 9/2016
– Joint Submission RTI, TwinOaks, eProsima
– Submission by PrismTech
• Presenta?ons in Reston focused on differences
– Goal was explore joining submissions
• Joining Submission stalled
– Technical details and documents a moving target
©2017 Real-Time Innova?ons, Inc.

5. Real-Time Innova?ons, Inc.
High Level Overview

6. Common approach: Agent-Based deployment model
XRCE
Agent
Global Data Space DDS/ RTPS
Client
Client
Client
DDS-XRCE
XRCE
Agent
Client
Client
Client
DDS App
DDS App
DDS App
DDS App
DDS App

7. Agent-Based deployment model
XRCE
Agent Client
XRCE Protocol
Global Data Space
DDS/ RTPS
Exis?ng DDS
Interoperability
Wire Protocol
New protocol defined
by the DDS-XRCE
specifica?on
Resource Constrained
Network
~ 40Kbps, 100Byte MTU
Resource Constrained
Device: ~ 100 KB RAM
Babery Powered
Sleep Cycles

8. Formal specifica?on of the Agent and Protocol
XRCE
Agent Client
XRCE Protocol
Global Data Space
DDS/ RTPS
XRCE
Protocol
DDS/RTPS
Protocol

9. Formal specifica?on of the Agent and Protocol
Client
XRCE Protocol
Global Data Space
DDS/ RTPS
HEADER MSG1 MSG2
XRCE Object Model
Message Structure
+
Interac=on Model
+

10. Real-Time Innova?ons, Inc.
XRCE Object Model

11. XRCE Agent
Object Model
class DDS-Mapping
DdsXrce::
DomainParticipant
DdsXrce::Publisher
DdsXrce::Subscriber
DdsXrce::
DataWriter
DdsXrce::DataReader
DdsXrce::Topic
DDS::DomainParticipantFactory
DDS::DomainParticipant
DDS::Publisher
DDS::Subscriber
DDS::TopicDescription
DDS::Topic
DDS::ContentFilteredTopic
DDS::DataReader
DDS::DataWriter
«value»
DdsXrce::Qos
«value»
DdsXrce::
QosProfile
Qos
+ DataReaderQos
+ DataWriterQos
+ DomainParticipantQos
+ PublisherQos
+ SubscriberQos
+ TopicQos
(from DDS)
DdsXrce::Application
«use»
«use»
«use»
«use»
«use»
«use»
«use»
«use»
0..*
«use»
«use»
«use»

12. XRCE Object Model highlights
• Uses UML to model Agent behavior
• XRCE Objects modeled as “resources”
– Addressable by their “name”
– CRUD opera?ons:
• Create/Read/Update(Write)/Delete
©2017 Real-Time Innova?ons, Inc.

13. ©2017 Real-Time Innova?ons, Inc.
class Application
DdsXrce::Application
+ create_participant(): ReturnStatus
+ update_participant(): ReturnStatus
+ delete_participant(): ReturnStatus
+ create_waitset(): ReturnStatus
+ update_waitset(): ReturnStatus
+ delete_waitset(): ReturnStatus
DdsXrce::DomainParticipant
DDS::DomainParticipant
DDS::
DomainParticipantFactory
DdsXrce::AccessController
+ check_permissions(): ReturnStatus
DdsXrce::WaitSet
«value»
DdsXrce::Type
«value»
DdsXrce::
QosProfile
DdsXrce::QosLibrary
«use»
«use»
«use»
«use»
«use»
0..*
«use»

14. ©2017 Real-Time Innova?ons, Inc.
class Participant
DdsXrce::Application
DdsXrce::DomainParticipant
+ register_type(): ReturnStatus
+ unregister_type(): ReturnStatus
+ create_topic(): ReturnStatus
+ update_topic(): ReturnStatus
+ delete_topic(): ReturnStatus
+ create_publisher(): ReturnStatus
+ update_publisher(): ReturnStatus
+ delete_publisher(): ReturnStatus
+ create_subscriber(): ReturnStatus
+ update_subscriber(): ReturnStatus
+ delete_subscriber(): ReturnStatus
+ get_topics(): ReturnStatus
+ get_publishers(): ReturnStatus
+ get_subscribers(): ReturnStatus
DDS::DomainParticipant
DDS::
DomainParticipantFactory
DdsXrce::AccessController
+ check_permissions(): ReturnStatus
DdsXrce::Topic DdsXrce::
Publisher
DdsXrce::
Subscriber
«value»
DdsXrce::Type
DdsXrce::
RegisteredType
«interface»
DdsXrce::Entity
+ name: string
«use»
«use»
0..*
«use»
«use»
«use»
«use»
«use»
typeName
registeredTypeName

15. ©2017 Real-Time Innova?ons, Inc.
class DataWriter
DdsXrce::DataWriter
+ create_instance(): ReturnStatus
+ update_instance(): ReturnStatus
+ delete_instance(): ReturnStatus
+ write(): ReturnStatus
DDS::DataWriter
DdsXrce::
Publisher
DdsXrce::Topic
DDS::Publisher
TopicDescription
DDS::Topic
«interface»
DdsXrce::Entity
+ name: string
«use»
«use»
«use»

16. ©2017 Real-Time Innova?ons, Inc.
class DataReader
DdsXrce::
Subscriber
DdsXrce::DataReader
+ get(): ReturnStatus
DdsXrce::Topic
- registeredTypeName: string
- topicName: string
DDS::
TopicDescription
DDS::Topic
DDS::DataReader
DDS::Subscriber
«interface»
DdsXrce::Entity
+ name: string
«use»
«use»
«use»
«use»

17. ©2017 Real-Time Innova?ons, Inc.
class AccessController
DdsXrce::AccessController
+ check_permissions(): ReturnStatus
«singleton»
DdsXrce::Root
DdsXrce::Application
DdsXrce::
Publisher
DdsXrce::
Subscriber
DdsXrce::
DomainParticipant
DdsXrce::Client
- clientAPIKey: string
0..*
«use»
«use»
«use»
«use»
0..*
0..*
1
0..*

18. Resource Names, Representa?on, and ID
• Each resource has:
– A name (addresses it within the Agent) and Context
• Example the type “ShapeType”
– A representa?on. Describes the resource
• Declara?ve way to configure Agent resources
• Not expected to go on the wire in most cases
• Example:
<struct name="ShapeStruct">
<member name="color" type="string"/>
<member name="x" type="int32"/>
<member name="y" type="int32"/>
<member name="shapesize" type="int32"/>
</struct>
– An ID. Provides (integer) shortcurt for Client to reference XRCE object/resource
• Note: Mul?ple representa?ons possible
– Currently leverages exis?ng XML formats
• As defined in various exis?ng DDS specifica?ons
– JSON and Binary are possible alterna?ves, if required
©2017 Real-Time Innova?ons, Inc.

19. Qos Representa?on
• Expect pre-defined profiles in Agent
– Just send QosPofile name
• Allow sending Qos using XML (or JSON format)
– JSON could be added
• Can configure complete set of DDS Qos policies
©2017 Real-Time Innova?ons, Inc.

20. Real-Time Innova?ons, Inc.
Message Structure

21. Message Header + Submessages
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XRCE Header Submessage Submessage Submessage
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| clientKey |
+---------------+---------------+---------------+---------------+
| sessionId | streamId | sequenceNr |
+---------------+-------+-------+---------------+---------------+
0 4 8 16 24 31
+---------------+---------------+---------------+---------------+
| smId | flags | resourceType | submessageLength |
+-------+-------+---------------+---------------+---------------+
| resourceId | count |
+-------+-------+---------------+---------------+---------------+
~ <payload – depends on smId & resourceType > ~
+-------+-------+---------------+---------------+---------------+
XRCE Header
Submessage

22. 12 kind of Sub Messages
©2017 Real-Time Innova?ons, Inc.
class Submessages
DdsXrce::Submessage
DdsXrce::
Submessage::
CREATE
DdsXrce::
Submessage::
GET_INFO
DdsXrce::
Submessage::
DELETE
DdsXrce::
Submessage::
INFO
DdsXrce::
Submessage::
STATUS
DdsXrce::
Submessage::
ACKNACK
DdsXrce::
Submessage::
HEARTBEAT
DdsXrce::Submessage::
SubmessageHeader
- submessageId: byte
- flags: byte
- submessageLength: short
DdsXrce::
Submessage::
DATA
DdsXrce::
Submessage::
FRAGMENT
DdsXrce::
Submessage::
READ_DATA
DdsXrce::
Submessage::
WRITE_DATA
DdsXrce::
Submessage::
RESET
1

23. Submessage kinds
©2017 Real-Time Innova?ons, Inc.
SubmessageId Value Note
CREATE 1 Client to Agent. Specifies resource Name or Representation
GET_INFO 2 Client to Agent.
DELETE 3 Client to Agent.
STATUS 4 Response to CREATE, UPDATE or DELETE.
INFO 5 Response to a GET_INFO. Contains information about an Xrce: Object.
WRITE_DATA 6 Client to Agent. Used to write data using a Xrce::DataWriter.
READ_DATA 7 Client to Agent. Used to read data using a Xrce::DataReader.
DATA 8 Agent to Client in response to a READ_DATA
ACKNACK 9 Bi-directional.
HEARTBEAT 10 Bi-directional.
RESET 11 Agent to Client.
FRAGMENT 12 Bi-directional

24. Example: DATA Message
©2017 Real-Time Innova?ons, Inc.
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| clientKey |
+---------------+---------------+---------------+---------------+
| sessionId | streamId | sequenceNr |
+---------------+---------------+---------------+---------------+
| DATA | flags | submessageLength |
+-------+-------+---------------+---------------+---------------+
| dataWriterResourceId | count |
+-------+-------+---------------+---------------+---------------+
~ <payload – data [0] ~
~ <payload – data [1] ~
~ <payload – data [count -1] ~
+---------------+-------+-------+---------------+---------------+
Payload format
Depends on flags
Serializa?on uses DDS-
XTYPES format

25. Example: DATA with Timestamp Message
©2017 Real-Time Innova?ons, Inc.
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| clientKey |
+---------------+---------------+---------------+---------------+
| sessionId | streamId | sequenceNr |
+---------------+---------------+---------------+---------------+
| DATA | flags | submessageLength |
+-------+-------+---------------+---------------+---------------+
| dataWriterResourceId | count |
+-------+-------+---------------+---------------+---------------+
~ <payload – common timestamp> ~
~ <payload – data [0] ~
~ <payload – data [1] ~
~ <payload – data [count -1] ~
+---------------+-------+-------+---------------+---------------+

26. Example: DATA with Timestamp Message
©2017 Real-Time Innova?ons, Inc.
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| clientKey |
+---------------+---------------+---------------+---------------+
| sessionId | streamId | sequenceNr |
+---------------+---------------+---------------+---------------+
| DATA | flags | submessageLength |
+-------+-------+---------------+---------------+---------------+
| dataWriterResourceId | count |
+-------+-------+---------------+---------------+---------------+
~ <payload – timestamp_base> ~
~ <payload – timestamp_offset[0] ~
~ <payload – data [0] ~
~ <payload – timestamp_offset[1] ~
~ <payload – data [1] ~
~ <payload – timestamp_offset[count -1] ~
~ <payload – data [count -1] ~
+---------------+-------+-------+---------------+---------------+

27. Real-Time Innova?ons, Inc.
Interac?on Model

28. ©2017 Real-Time Innova?ons, Inc.
sd Client Agent
XRCE AgentXRCE Client
RESOURCE_STATUS(DataWriter)
RESOURCE_STATUS(Session)
ASSERT_RESOURCE(Session)
WRITE_DATA(DataWriter)
WRITE_DATA(DataWriter)
ASSERT_RESOURCE(DataWriter)
DELETE_RESOURCE(DataWriter, Session)
WRITE_DATA(DataWriter)
ASSERT_RESOURCE iden?fies (preconfigured)
resource by means its Name and retrieves
resourceId
Note: Using CREATE instead of
ASSERT_RESOURCE can create the resource
from its representa?on

29. ©2017 Real-Time Innova?ons, Inc.
sd Minimal Publisher
XRCE Client XRCE Agent
DELETE_RESOURCE()
WRITE_DATA()
ASSERT_RESOURCE(Session)
WRITE_DATA()
RESOURCE_STATUS()
WRITE_DATA()
The resourceId for specific objects (e.g. a
DataWriter) can also be pre-configured.
This saves having to ASSERT it.

30. ©2017 Real-Time Innova?ons, Inc.
sd Minimal Subscriber
XRCE Client XRCE Agent
DATA()
DATA()
ASSERT_RESOURCE(Session)
DELETE_RESOURCE()
DATA()
CONFIG_READ()

31. Real-Time Innova?ons, Inc.
Transport Model

32. Transport requirements
• Packet-oriented or connec?on oriented
• It does not need to provide reliability
– Messages may be dropped
• It does not need to provide ordering
– Messages may arrive out of order
• It does not need to provide no?fica?on of dropped messages
• Must support transport-level security (e.g. TLS or DTLS)
©2017 Real-Time Innova?ons, Inc.

33. Reliability
• Reliability per stream, mul?ple (256) streams
– Buil?n streams 0-5 are present on every session
– Even-numbered streamIds are best-efforts.
– Reliable streams use HB/ ACKNACK protocol
• Similar to DDS/RTPS
• Mul?ple Writers can share same stream as configured by client
• Mul?ple Readers can also share the same stream
©2017 Real-Time Innova?ons, Inc.

34. Real-Time Innova?ons, Inc.
Security

35. Security approach
• Assumes transport-level security is in place
• Message Header includes (4-Byte) secret ClientKey
• Agent uses ClientKey to determine client permissions and
Iden?ty
– Leverages DDS-Security iden?ty and permissions documents
– Proxy DDS DomainPar?cipant are exclusive to a single Client
©2017 Real-Time Innova?ons, Inc.

36. Real-Time Innova?ons, Inc.
Other

37. Other points
• Reuses CDR (XTYPES) serializa?on
• Serialized Resource format described in IDL
• Data serialized using CDR. Data can be relayed serialized
• Supports fragmenta?on
• Supports mul?ple concurrent sessions
• Full “DDS reachability”
– Any Qos available in DDS
– Any data type available in DDS-XTYPES
©2017 Real-Time Innova?ons, Inc.

38. Real-Time Innova?ons, Inc.
RFP equirements

39. RFP mandatory requirements
©2017 Real-Time Innova?ons, Inc.
RFP requirement Submission
DDS-XRCE protocol between a Client and an Agent YES
DDS-XRCE clients to be perceived by DDS Par?cipants as at least
minimum profile DDS Par?cipants
YES, the can full DDS
Par?cipants, not limited to
minimum profile
DDS-XRCE protocol overhead, when sending or receiving user data, shall
not exceed 24 bytes per packet
YES, less than 20 bytes
Address clients opera?ng with sleep/wakeup periods YES, state maintained by
Agent, “stateless” messages
DDS-XRCE protocol for the UDP/IP transport.
DDS-XRCE protocol for the TCP/IP transport
YES, protocol is independent
from transport.

40. Real-Time Innova?ons, Inc.
Path forward
Does it s?ll make sense to try to merge?

41. Path forward
• Revised Submission in September 2017
• Con?nue inves?ga?on of possible combined submission
©2017 Real-Time Innova?ons, Inc.