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Message Header

  • Protocol Version: 3.0
  • Release Date: January 20, 2017

Header Fields

Data Type Description
V uint16 Header version number (2 for version 2 and 3)
TYPE char[12] Type name of data
DEVICE_NAME char[20] Unique device name
TIME_STAMP uint64 Timestamp or 0 if unused
BODY_SIZE uint64 Size of the body in bytes
CRC uint64 64 bit CRC for body data

Header version number

The header version number field specifies the Header Version. Please note that the Header Version is not the same as the Protocol Version, as the Protocol Version reflects not only the format of the header but also the available message types. Two header versions exist as of Protocol Version 3.x:

  • Header Version 1 is used in Protocol Version 1 and 2.
  • Header Version 2 is used in Protocol Version 3.x.

The major difference between header versions 1 and 2 is the mechanism to include the extended header and meta data in the message. This feature has been introduced in Protocol Version 3.

Type name

The type name field is an ASCII character string specifying the type of the data contained in the message body e.g. "TRANSFORM". The length of the type name must be within 12 characters.

Device name

The device name field contains an ASCII character string specifying the name of the the message.


The timestamp field contains a 64-bit timestamp indicating when the data is generated. Please refer Timestamp for the format of the 64-bit timestamp.

Body size

The body size field contains the size of the body in bytes. The Body is a section of the OpenIGTLink message following the Header. Until Version 2, the Body only contains the message content, and hence many old programs assumed that the Body size represents the content size. Since Version 3, the Body is redefined as a set of Extended Header, Content, and Metadata. The content size must be calculated by BODY_SIZE - (EXT_HEADER_SIZE + METADAT_SIZE).


The 64-bit CRC used in OpenIGTLink protocol is based on ECMA-182 standard. An example code is available in igtl_util.c in the OpenIGTLink library.

Extended Header Fields

Data Type Description
EXT_HEADER_SIZE uint16 Length of Extended Header section
METADATA_SIZE uint16 Length of Metadata section
MSG_ID uint32 Message ID
(Reserved) Uint32 (Reserved)

Metadata Fields

Data Type Description
INDEX_COUNT uint16 Number of meta data
META_HEADER_0 META_HEADER Header for metadata 0
... ... ...
META_DATA_0 META_DATA Content for metadata 0
... ... ...
Data Type Description
KEY_SIZE uint16 Size of key (bytes)
VALUE_ENCODING uint16 Character encoding type for value as MIBenum value.
VALUE_SIZE uint32 Size of value (bytes)
Data Type Description
KEY uint16[KEY_SIZE] Key (ASCII)
VALUE uint16[VALUE_SIZE] Value (Encoded according to VALUE_ENCODING)
  • For MIBenum character encoding, please refer IANA Character Sets. US-ASCII (ANSI-X3.4-1968; MIBenum = 3) is strongly recommended.

Byte Order

Big endian should be used for all numerical values (version, body size, crc). Unused spaces are padded with 0 (binary).