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Overview of the DDS-XRCE specification

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Gerardo Pardo-Castellote

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.

Software
Related topics:
Wireless Sensor Networks
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Gerardo Pardo-Castellote, Ph.D. CTO, Real-Time Innovations, Inc. DDS-XRCE Specification Overview DDS for Extremely Resource Contained Networks
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.
Outline • Motivation • Big Picture • Message Structure • Interaction Model • Security • Deployments • Transport Model • Conformance Profiles • Conclusion ©2017 Real-Time Innovations, Inc.
Real-Time Innovations, Inc. Motivation
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
Overview of the DDS-XRCE specification
Real-Time Innovations, Inc. Big Picture
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
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
Formal specification of the Agent and Protocol XRCE AgentClient XRCE Protocol Global Data Space DDS/ RTPS XRCE Protocol DDS/RTPS Protocol
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 +
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»
©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»
©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»
©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»
©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»
©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»
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.
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.
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.
Real-Time Innovations, Inc. Message Structure
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
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
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
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
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
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 +---------------+----------------+----------------+----------------+
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]
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)
Real-Time Innovations, Inc. Interaction Model
©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
©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.
©2017 Real-Time Innovations, Inc. sd Minimal Subscriber XRCE Client XRCE Agent DATA() DATA() ASSERT_RESOURCE(Session) DELETE_RESOURCE() DATA() CONFIG_READ()
Real-Time Innovations, Inc. Security
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.
Real-Time Innovations, Inc. XRCE Deployments
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
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
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
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
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
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
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
Real-Time Innovations, Inc. Transport Model
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.
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.
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.
Real-Time Innovations, Inc. Conformance profiles
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
Real-Time Innovations, Inc. Conclusion
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.

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Overview of the DDS-XRCE specification

  • 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.
  • 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
  • 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.
  • 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)
  • 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()
  • 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.
  • 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
  • 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.
  • 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
  • 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.