SpreadProtocol¶

This wiki page describes the protocol used by the spread-based connectors.

Data Format¶

Data exchanged on spread is encoded using Google protocol buffers. A spread message always contains a notification (source:/trunk/protocol/RSBProtocol/Protocol.proto) as elementary communication unit. Descriptions of the Notification contents are given as comments on the descriptor file.

Fragmentation¶

Because spread has a message size limit, a single Notification may not be sufficient to transport a whole event with a huge amount of user data. Hence, events may be encoded in several Notification s sent subsequently. In this case the multiple Notification objects are constructed according to the following rules:

• The size of individual fragments (i.e. the values of Notification.data) must not exceed 100,000 octets
• Fragment number are in the range [0, NUMBER-OF-FRAGMENTS - 1]
• Most field values are repeated in all Notification s, in particular:
  • The id of the event (Notification.id field)
  • The number of Notification s into which the event has been split (Notification.num_data_parts field)
• The two fields which differ among Notification s for one event are:
  • The index of the data chunk encoded in the Notification (Notification.data_part field).
  • The user data (Notification.data field) are split.

Hierarchical Bus¶

The hierarchical bus is created by sending each message to a group corresponding to its scope as well as groups corresponding to all super scopes including "/" (multigroup mulitcast).

Example

super-scopes(/foo/bar/, include-self? = yes) = /, /foo/, /foo/bar/

Group Names¶

Group names are created by applying the following steps to the fully formal scope string representation (including trailing slash):

1. Hash the scope string using MD5.
2. Convert the 16 bytes of output to a 32 character string by concatenating the zero-padded hex (base-16) representations of the individual bytes.
   Letters of the hex representation have to be lower case.
3. Remove the final character of the hex representation of the hash.
   (Since spread group names can only be 32 bytes long including the 0-terminator)

Example

/         -> "6666cd76f96956469e7be39d750cc7d\0" 
/foo/     -> "4f87be8f6e593d167f5fd1ab238cfc2\0" 
/foo/bar/ -> "1c184f3891344400380281315d9e738\0" 

Quality of Service¶

The following table explains how the 2D RSB QoS settings are mapped to spread message types.

	UNRELIABLE	RELIABLE
UNORDERED	UNRELIABLE_MESS	RELIABLE_MESS
ORDERED	FIFO_MESS	FIFO_MESS