4.12. Message Integrity Checking

Connext DDS Micro uses the DDS-I/RTPS protocol for communication between DDS applications, and RTPS messages are sent and received by a transport. When an RTPS message is sent across a communication link, such as Ethernet, it is possible that some bits may change value. These errors may cause communication failures or incorrect data to be received. In order to detect these types of errors, transports such as UDP often include a checksum to validate the integrity of the data: a sender adds the checksum to the transmitted data and the receiver validates that the calculated checksum for the received data matches the checksum received from the sender. If the checksums are different, a data corruption has occurred.

By default, Connext DDS Micro relies on the underlying transport, such as UDP, to handle data integrity checking. However, the underlying transport may not provide sufficient integrity checking, or may itself introduce errors that Connext DDS Micro must be able to detect regardless of the transport.

In order to address both of these scenarios for any transport, Connext DDS Micro supports RTPS message integrity checking by adding a checksum to the RTPS message itself. This chapter describes the setup and default options to access this feature.

For information on how to write custom checksum functions, please refer to RTPS.

4.12.1. RTPS Checksum

Connext DDS Micro implements checksum validation on a complete RTPS message. A typical RTPS message without a checksum has the following structure:

+--------+------------+-------------------------+------------+
| Header | Submessage | ...... submessages .... | Submessage |
+--------+------------+-------------------------+------------+

When the message integrity checking feature is enabled, the structure of the RTPS message changes as illustrated below:

+--------+------------+------------+------------------+------------+
| Header | Checksum   | Submessage | .. submessages ..| Submessage |
+--------+------------+------------+------------------+------------+

The sender calculates the checksum for the entire message with a checksum field set to 0 and places the result in the checksum field.

The receiver saves the the received checksum, sets the received checksum field to 0, and calculates the checksum for the entire message. It then compares the calculated checksum with the received checksum. If the checksums differ, the entire RTPS message is considered corrupted.

Note that the checksum is used only for error detection and not for error correction.

4.12.2. Configurations

You can configure your application to define which algorithms to use and validate as well as the requirements enforced by the participant when communicating with other participants using the DDS_WireProtocolQosPolicy.

Configuring the message integrity checking consists of the two parts:

  1. Selecting the checksum algorithm.

  2. Configuring how a participant applies the checksums.

4.12.2.1. Selecting a checksum algorithm

Connext DDS Micro supports three built-in algorithms and can be configured to use any of the following algorithms:

  1. DDS_CHECKSUM_BUILTIN32: CRC-32 As defined by ISO/IEC 13239:2002.

  2. DDS_CHECKSUM_BUILTIN64: CRC-64 As defined by ISO/IEC 13239:2002.

  3. DDS_CRC_BUILTIN128: MD5 Message Digest

The CRC functions have the following properties:

Checksum

Polynom

Initial Value

Input Reflected

Output Reflected

XOR Value

CRC-32

0x04c11db7

2^32 - 1

true

true

2^32 - 1

CRC-64

0x1b

2^64 - 1

true

true

2^64 -1

In addition, four custom algorithms can implemented and used:

  1. DDS_CHECKSUM_CUSTOM32

  2. DDS_CHECKSUM_CUSTOM64

  3. DDS_CHECKSUM_CUSTOM128

  4. DDS_CHECKSUM_CUSTOM256

Please refer to RTPS for information on how to implement custom checksum functions.

4.12.2.2. Configuring the DDS DomainParticipant

The RTPS message integrity feature is configured in the DDS_WireProtocolQosPolicy for a participant. This QoS determines which RTPS checksum should be allowed, and if checksums should be sent and/or validated.

The following three fields determine how a participant uses RTPS checksums:

  • compute_crc - This configures the participant to send a checksum with each RTPS message. Which checksum to send is determined by computed_crc_kind.

  • check_crc - This configures the participant to verify the checksum in each received RTPS message if the checksum is present. If the checksum is valid, the message is accepted; otherwise, the message is dropped. If a message is received without a checksum, it is accepted and processed.

  • require_crc - This configures the participant to require that a checksum is present in the receiving packet. Messages without a checksum are dropped without further processing. Note that this option is orthogonal to the check_crc options. This option only requires that a checksum is included, it does not validate it. To validate and only accept messages with a checksum, both check_crc and require_crc must be true.

The following two fields determine which checksums are used:

  • computed_crc_kind - The checksum type to include in each RTPS message when compute_crc is true.

  • allowed_crc_mask - A mask of all checksum algorithms that the participant can verify. This allows the participant to receive messages from other participants with a different computed_crc_kind. A participant will ignore a participant that is sending a checksum that it cannot validate.

For example, the following snippet shows how to configure the participant to:

  • Send all messages (except the participant announcements; see the Participant Discovery and Participant Compatibility section below) with DDS_CHECKSUM_BUILTIN64.

  • Accept DDS_CHECKSUM_BUILTIN32, DDS_CHECKSUM_BUILTIN64, and DDS_CHECKSUM_BUILTIN128 algorithms.

    struct DDS_DomainParticipantQos dp_qos =
                                  DDS_DomainParticipantQos_INITIALIZER;

dp_qos.protocol.computed_crc_kind =  DDS_CHECKSUM_BUILTIN64;

dp_qos.protocol.allowed_crc_mask = DDS_CHECKSUM_BUILTIN32
                                   | DDS_CHECKSUM_BUILTIN32
                                   | DDS_CHECKSUM_BUILTIN64;

4.12.3. Participant Discovery and Participant Compatibility

Connext DDS Micro ensures that participants establish communication with each other only when they have compatible checksum configurations. If compute_crc is true, all messages sent from the participant are protected by a checksum. Since each participant can use a different type of checksum, a mechanism is required to ensure that participants are compatible during discovery.

To bootstrap this mechanism, all participant announcements (if compute_crc is set to true) include a checksum of type DDS_CHECKSUM_BUILTIN32. The participant announcement carries information about the computed_crc_kind (the checksum kind used by the participant) and the allowed_crc_mask (the checksum kinds understood by the participant), and whether or not the participant requires a checksum for each RTPS message (if require_crc is set to true). Please note that messages with DDS_CHECKSUM_BUILTIN32 checksum are always accepted to enable discovering new participants.

For a Participant (A) to match with another Participant (B), the computed_crc_kind of Participant (B) must be a strict subset of the allowed_crc_mask of Participant (A) and vice versa. If Participant (B) does not send a checksum (compute_crc is set to false), it can only match Participant (A) if it does not set require_crc to true.

4.12.4. Interoperability with Connext DDS Professional

Connext DDS Professional supports a CRC 32-bit checksum. However, the RTPS submessage used by Connext DDS Professional to include a checksum is different from the one used by Connext DDS Micro and what has been standardized by the OMG. Connext DDS Micro always accepts Connext DDS Professional’s CRC32 and treats it as a DDS_CHECKSUM_BUILTIN32. However, in order to enable interoperability with Connext DDS Professional and enable Connext DDS Professional to validate the checksum, it is necessary to change the transmit mode of Connext DDS Micro. Two transmit modes are available:

  • RTPS_CRC_TXMODE_OMG - Use the standard method as defined by the OMG. This is the default mode. The checksums sent by Connext DDS Micro are not understood by Connext DDS Professional, and Connext DDS Professional will accept the messages as not having a CRC32.

  • RTPS_CRC_TXMODE_RTICRC32 - CRC32 Mode. This mode sets the computed_crc_kind to DDS_CRC_BUILTIN32. The checksum sent by Connext DDS Micro is understood by Pro. Use this option only if the Connext DDS Professional application in your system needs checksum validation and has set check_crc to true.