The document provides an overview of the DDS-XRCE specification, which defines an Agent-Client communication model to enable the use of the DDS data distribution service (DDS) in extremely resource-constrained networks. It describes the motivation for DDS-XRCE and its key aspects, including the message structure, interaction model, supported deployment scenarios, and how it provides security through the use of a client key.

1. Gerardo Pardo-Castellote, Ph.D.
CTO, Real-Time Innovations, Inc.
DDS-XRCE Specification Overview
DDS for Extremely Resource Contained Networks

2. New OMG Specification
• The DDS-XRCE specification was adopted March 2018
• Documents are available from the OMG at:
https://www.omg.org/spec/DDS-XRCE/
• It is considered a Beta specification to be finalized by March 2019
©2017 Real-Time Innovations, Inc.

3. Outline
• Motivation
• Big Picture
• Message Structure
• Interaction Model
• Security
• Deployments
• Transport Model
• Conformance Profiles
• Conclusion
©2017 Real-Time Innovations, Inc.

4. Real-Time Innovations, Inc.
Motivation

5. DDS is Being Extended to the very Edge
• Users want to leverage the benefits of the DDS Data-Bus to
applications at the edge of the network, including Sensor Networks
• These systems are constrained in many dimensions: CPU, memory,
power, connectivity, transport payload & bandwidth
• The constraints make it not possible make the edge nodes direct
peer-to-peer participants on the DDS Domain and run DDS-RTPS
• DDS-XRCE solves this problem by defining a new Client-to-Agent
allowing the use of an Agent to communicate with a DDS Domain

7. Real-Time Innovations, Inc.
Big Picture

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

9. Agent-Based deployment model
XRCE
AgentClient
XRCE Protocol
Global Data Space
DDS/ RTPS
Existing DDS
Interoperability Wire
Protocol
New protocol defined by
the DDS-XRCE
specification
Resource Constrained
Network
~ 40Kbps, 100Byte MTU
Resource Constrained
Device: ~ 100 KB RAM
Battery Powered
Sleep Cycles

10. Formal specification of the Agent and Protocol
XRCE
AgentClient
XRCE Protocol
Global Data Space
DDS/ RTPS
XRCE Protocol DDS/RTPS
Protocol

11. Formal specification of the Agent and Protocol
Client
XRCE Protocol
Global Data Space
DDS/ RTPS
HEADER MSG1 MSG2
XRCE Object Model
Message Structure
+
Interaction Model
+

12. 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»
0..*
«use»
«use»
«use»
«use»
«use»

13. ©2017 Real-Time Innovations, Inc.
classApplication
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
«value»
DDSXRCE::Type
«value»
DDSXRCE::
QosProfile
DDSXRCE::QosLibrary «use»
«use»
«use»
«use»
«use»
0..*
«use»

14. ©2017 Real-Time Innovations, Inc.
classParticipant
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
0..*
«use»
«use»
«use»
«use»
registeredTypeName
«use»
typeName
«use»
«use»

15. ©2017 Real-Time Innovations, 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 Innovations, Inc.
classDataReader
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 Innovations, Inc.
classAccessController
DDSXRCE::AccessController
+ check_permissions(): ReturnStatus
«singleton»
DDSXRCE::Root
DDSXRCE::Application
DDSXRCE::
Publisher
DDSXRCE::
Subscriber
DDSXRCE::
DomainParticipant
DDSXRCE::ProxyClient
- clientAPIKey: string
0..*
«use»
0..*
0..*
1
«use»
«use»
«use»

18. Resource Names, Representation, and ID
• Each resource has:
– A name (addresses it within the Agent) and Context
• Example the type “ShapeType”
– A representation. Describes the resource
• Declarative way to describe the Agent resource
• Not expected to go on the wire in most cases
– An ID. Provides (integer) shortcurt for Client to reference XRCE object/resource
• Multiple representations supported: NAME, XML, BINARY
©2017 Real-Time Innovations, Inc.

19. Qos Representation
• Typical: pre-defined profiles in Agent
– Name-Representation -> QosPofile name
• Allow exchanging Qos between client and Agent:
– XML Representation. Uses XML (or future JSON format)
• Can configure complete set of DDS Qos policies
• Re-uses DDS-XTYPES
– Binary Representation
• Allow sending a subset of the Qos using a BINARY representation
• Reuses XCDR
• Example:
<qos_profile name="MyProf" base_name="ExistingProfile " >
<datawriter_qos>
<history>
<depth>10</depth>
</history>
</datawriter_qos>
</qos_profile>
©2017 Real-Time Innovations, Inc.

20. Type Representation
• Typical: types pre-defined profiles in Agent
• Name-Representation -> QosPofile name
• Allow exchanging Types between client and Agent:
– XML Representation. Uses XML (or future JSON format)
• Can configure complete set of DDS Types policies
• Re-uses DDS-XTYPES
– Binary Representation (possible extension)
• Allow exchange a any type information using a BINARY representation
• Reuses XCDR
• Example type description using XML representation:
<struct name="ShapeStruct">
<member name="color" type="string"/>
<member name="x" type="int32"/>
<member name="y" type="int32"/>
<member name="shapesize" type="int32"/>
</struct>
©2017 Real-Time Innovations, Inc.

21. Real-Time Innovations, Inc.
Message Structure

22. Message Header + Submessages
©2017 Real-Time Innovations, Inc.
XRCE Header Submessage Submessage Submessage
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| sessionId | streamId | sequenceNr |
+---------------+-------+-------+---------------+---------------+
| clientKey (optional is sessionId > 128) |
+---------------+---------------+---------------+---------------+
0 4 8 16 24 31
+---------------+---------------+---------------+---------------+
| submessageId | flags | submessageLength |
+---------------+---------------+---------------+---------------+
~ <payload – depends on smId & resourceType> ~
+---------------+---------------+---------------+---------------+
XRCE Header
Submessage

23. 13 kind of Sub Messages
©2017 Real-Time Innovations, Inc.
classSubmessages
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
DDSXRCE::
Submessage::
CREATE_CLIENT
1

24. Submessage kinds
©2017 Real-Time Innovations, 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 Bi-directional
FRAGMENT 12 Bi-directional

25. Example: DATA Message
©2017 Real-Time Innovations, Inc.
0 8 16 24 31
+---------------+--------+-------+----------------+----------------+
| session_id | stream_id | sequenceNr | 4
+-------+-------+----------------+----------------+----------------+
| DATA | flags | submessageLength | 8
+---------------+----------------+----------------+----------------+
| request_id | object_id | 12
+---------------+----------------+----------------+----------------+
+ XCDR_Serialization(foo) + 16
+---------------+--------+-------+----------------+----------------+
Payload format
Depends on flags
Serialization uses DDS-
XTYPES format

26. Example: Creating a Client
©2017 Real-Time Innovations, Inc.
0 8 16 24 31
+---------------+---------------+---------------+---------------+
| 0x80 | 0x00 | 0x00 | 4
+-------+-------+---------------+---------------+---------------+
| CREATE_CLIENT | flags | submessageLength | 8
+-------+-------+---------------+---------------+---------------+
| request_id | object_id | 12
+-------+-------+---------------+---------------+---------------+
| xrce_cookie | 16
+-------+-------+-------+-------+-------+-------+-------+-------+
| xrce_version | xrce_vendor_id | 20
+-------+-------+---------------+---------------+---------------+
| client_timestamp.seconds | 24
+ +
| client_timestamp.nanoseconds | 26
+-------+-------+---------------+---------------+---------------+
| client_key | 30
+-------+-------+---------------+---------------+---------------+
| session_id |
+-------+-------+
Built-in security
mechanism using
ClientKey

27. Example: DATA as a sequence
©2017 Real-Time Innovations, Inc.
0 8 16 24 31
+---------------+----------------+----------------+----------------+
| session_id | stream_id | sequenceNr | 4
+-------+-------+----------------+----------------+----------------+
| READ_DATA | flags | submessageLength | 8
+---------------+----------------+----------------+----------------+
| request_id | object_id | 12
+---------------+----------------+----------------+----------------+
| is_present=0 | FORMAT_DATA_SEQ| max_samples | 16
+---------------+----------------+----------------+----------------+
| max_elapsed_time | 20
+---------------+----------------+----------------+----------------+
| max_rate | 24
+---------------+----------------+----------------+----------------+

28. Writer Reliability
©2017 Real-Time Innovations, Inc.
Initial
AllMessagesAcknowledged MessagesNotFullyAcked
write
/HighestSentSequenceNumber++
receive(ACKNACK)
/save
HigestAcknowledgedSeqNumber
repair SequenceNumber in NACK
[HeartBeatPeriod]
/Send(HeartBeat, HighestSentSequenceNumber)
write
/HighestSentSequenceNumber++
[HiguestReceivedSeqNumber ==HiguestSentSequenceNumber]

29. Reader Reliability
©2017 Real-Time Innovations, Inc.
Initial
ReceivedAllMessages
MissingMessageFromSender
Choice
/ HiguestReceivedSequenceNumber :=0
[HiguestAnnouncedSequenceNumber <=HiguestReceivedSequenceNumber]
receive(HeartBeat, HigestAnnouncedSequenceNumber)
[HiguestAnnouncedSequenceNumber >HiguestReceivedSequenceNumber]
receive(HeartBeat, HigestAnnouncedSequenceNumber)
[XRCEClient && NackPeriod]
/Send(ACKNACK)

30. Real-Time Innovations, Inc.
Interaction Model

31. ©2017 Real-Time Innovations, 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 identifies (preconfigured)
resource by means its Name and retrieves
resourceId
Note: Using CREATE instead of ASSERT_RESOURCE
can create the resource from its representation

32. ©2017 Real-Time Innovations, 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.

33. ©2017 Real-Time Innovations, Inc.
sd Minimal Subscriber
XRCE Client XRCE Agent
DATA()
DATA()
ASSERT_RESOURCE(Session)
DELETE_RESOURCE()
DATA()
CONFIG_READ()

34. Real-Time Innovations, 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 Identity
– Leverages DDS-Security identity and permissions documents
– Proxy DDS DomainParticipant are exclusive to a single Client
©2017 Real-Time Innovations, Inc.

36. Real-Time Innovations, Inc.
XRCE Deployments

37. Six deployment scenarios
• XRCE Client to DDS communication
• XRCE Client to Client via DDS
• Client-to-Client communication brokered by an Agent
• Federated deployment
• Direct Peer-to-Peer communication between clients
• Combined deployment

38. XRCE Client to DDS communication
©2017 Real-Time Innovations, Inc.
Global Data Space
XRCE
Agent
DDS-RTPS
Protocol
XRCE
Client
DDS
DomainParticipant
DDS-RTPS
XRCE Protocol

39. XRCE Client to Client via DDS
©2017 Real-Time Innovations, Inc.
Global Data Space
XRCE
Agent
XRCE
Client
XRCE
Agent
XRCE
Client
XRCE Protocol
XRCE Protocol
DDS-RTPS
Protocol
DDS-RTPS
Protocol

40. Client-to-Client communication brokered by an Agent
©2017 Real-Time Innovations, Inc.
XRCE Protocol
Global Data Space
XRCE
Agent
XRCE
Client
XRCE
Client
DDS-RTPS
Protocol

41. Federated deployment
©2017 Real-Time Innovations, Inc.
Global Data Space
XRCE
Agent
XRCE
Client
XRCE
Agent
XRCE
Client
DDS-RTPS
Protocol
XRCE
Client
XRCE
Protocol
XRCE
Agent

42. Direct Peer-to-Peer communication between clients
• Not an RFP requirement
• Protocols supports it without any
additions
• Applications “opt-in” by instantiating:
– Embedded XRCE Agent
– XRCE Client per matched peer
• Maybe implemented without extra
objects. Just the state needs to be
kept.
©2017 Real-Time Innovations, Inc.
XRCE
Agent
XRCE
Client
XRCE
Client
XRCE
Agent
XRCE
Client
XRCE
Client
XRCE Protocol
XRCE
Agent
XRCE
Client
XRCE
Client

43. Combined deployment
©2017 Real-Time Innovations, Inc.
Global Data Space
XRCE
Agent
XRCE
Client
DDS-RTPS
Protocol
XRCE
Client
XRCE
Protocol
XRCE
Agent
DDS
DomainParticipant
DDS
DomainParticipant
XRCE
Agent
XRCE
Client
XRCE
Client
XRCE
Agent
XRCE
Client
XRCE
Client

44. Real-Time Innovations, Inc.
Transport Model

45. Transport Mappings
• Specifies common requirements for XRCE Transports
• Specifies mapping to UDP and TCP
• Pattern for additional Transport mappings. Define:
– Transport Locators
– Connection Establishment
– Message Envelopes
– Agent Discovery
©2017 Real-Time Innovations, Inc.

46. Transport requirements
• Packet-oriented or connection 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 notification of dropped messages
• Must support transport-level security (e.g. TLS or DTLS)
©2017 Real-Time Innovations, Inc.

47. Reliability
• Reliability per stream, multiple (256) streams
– Builtin streams 0-5 are present on every session
– Even-numbered streamIds are best-efforts.
– Reliable streams use HB/ ACKNACK protocol
• Similar to DDS/RTPS
• Multiple Writers can share same stream as configured by client
• Multiple Readers can also share the same stream
©2017 Real-Time Innovations, Inc.

48. Real-Time Innovations, Inc.
Conformance profiles

49. Conformance profiles
Profile Description
Read Access Client can read any DDS
Write Access Client can write any DDS
Configure Entities Client can configure/define DDS Entities dynamically
Agent can have Qos configured/defined remotely
Configure Qos Client can configure/define DDS Qos dynamically
Agent can have Qos configured/defined remotely
Configure Types Client can define DDS Type dynamically
Agent can have Qos configured/defined remotely
Discovery Client can discover Topics and Types available on DDS GDS
File-based Config Agent can be configured via XML files
UDP Transport Implements XRCE on UDP

50. Real-Time Innovations, Inc.
Conclusion

51. DDS-XRCE Conclusions
• New DDS-XRCE specification adopted by OMG in March 2018
– Considered Beta until March 2019
– https://www.omg.org/spec/DDS-XRCE/
• DDS-XRCE extends DDS to resource-constrained environments
– Memory constraints
– CPU constraints (e.g. Micro-controllers)
– Power Constraints (battery power and sleep cycles)
– Transport constraints (ZigBee, BLE, …)
• Uses a Client/Agent architecture
– Devices connect to DDS Domain via an Agent
• DDS-XRCE SDK allows full access to DDS Topic data from the resource-
constrained device using the Agent as relay
©2017 Real-Time Innovations, Inc.