ROS/Connection Header
The ROS connection header contains important metadata about a connection being established, including typing information and routing information. How it is exchanged depends on the ROS transport being used.
See also:
Encoding Format
4-byte length + [4-byte field length + field=value ]*
All length fields are little-endian integers. Field names and values are strings.
1. Example encoding of a published message
Raw hex output from rostopic pub /chatter std_msgs/String "hello" (new lines and indenting added for readability):
b0 00 00 00 20 00 00 00 6d 65 73 73 61 67 65 5f 64 65 66 69 6e 69 74 69 6f 6e 3d 73 74 72 69 6e 67 20 64 61 74 61 0a 0a 25 00 00 00 63 61 6c 6c 65 72 69 64 3d 2f 72 6f 73 74 6f 70 69 63 5f 34 37 36 37 5f 31 33 31 36 39 31 32 37 34 31 35 35 37 0a 00 00 00 6c 61 74 63 68 69 6e 67 3d 31 27 00 00 00 6d 64 35 73 75 6d 3d 39 39 32 63 65 38 61 31 36 38 37 63 65 63 38 63 38 62 64 38 38 33 65 63 37 33 63 61 34 31 64 31 0e 00 00 00 74 6f 70 69 63 3d 2f 63 68 61 74 74 65 72 14 00 00 00 74 79 70 65 3d 73 74 64 5f 6d 73 67 73 2f 53 74 72 69 6e 67 09 00 00 00 05 00 00 00 68 65 6c 6c 6f
The output can be translated as:
- The message header length is 176 bytes (b0 00 00 00, length field encoding is little-endian).
- The message_definition field length is 32 bytes (20 00 00 00):
6d 65 73 73 61 67 65 5f 64 65 66 69 6e 69 74 69 6f 6e 3d 73 74 72 69 6e 67- m e s s a g e _ d e f i n i t i o n = s t r i n g- 20 64 61 74 61 0a 0a d a t a \n \n
- The callerid field length is 37 bytes (25 00 00 00):
63 61 6c 6c 65 72 69 64 3d 2f 72 6f 73 74 6f 70 69 63 5f 34 37 36 37 5f 31 33- c a l l e r i d = / r o s t o p i c _ 4 7 6 7 _ 1 3- 31 36 39 31 32 37 34 31 35 35 37 1 6 9 1 2 7 4 1 5 5 7
- The latching field length is 10 bytes (0a 00 00 00):
6c 61 74 63 68 69 6e 67 3d 31 l a t c h i n g = 1
- The md5sum field length is 39 bytes (27 00 00 00):
6d 64 35 73 75 6d 3d 39 39 32 63 65 38 61 31 36 38 37 63 65 63 38 63 38 62 64- m d 5 s u m = 9 9 2 c e 8 a 1 6 8 7 c e c 8 c 8 b d- 38 38 33 65 63 37 33 63 61 34 31 64 31 8 8 3 e c 7 3 c a 4 1 d 1
- The topic field length is 14 bytes (0e 00 00 00):
74 6f 70 69 63 3d 2f 63 68 61 74 74 65 72 t o p i c = / c h a t t e r
- The message type length is 20 bytes (14 00 00 00):
74 79 70 65 3d 73 74 64 5f 6d 73 67 73 2f 53 74 72 69 6e 67 t y p e = s t d _ m s g s / S t r i n g
- The message body length is 9 bytes (09 00 00 00).
- The first field of the std_msgs/String message (data) is 5 bytes (05 00 00 00):
68 65 6c 6c 6f h e l l o
Fields
See TCPROS information on the specific fields used when creating TCPROS connections.
- callerid: name of node sending data
- topic: name of the topic the subscriber is connecting to
- service: name of service the client is calling
- md5sum: md5sum of the message type
- type: message type
- message_definition: full text of message definition (output of gendeps --cat)
- error: human-readable error message if the connection is not successful
- persistent: sent from a service client to a service. If '1', keep connection open for multiple requests.
- tcp_nodelay: sent from subscriber to publisher. If '1', publisher will set TCP_NODELAY on socket if possible
- latching: publisher is in latching mode (i.e. sends the last value published to new subscribers)