The protocol description is maintained as part of source code for the C/C++ library. Please refer to OpenIGTLink Protocol Description in GitHub for the up-to-date description.

OpenIGTLink is an open-source network communication interface specifically designed for image-guided interventions. It aims to provide a plug-and-play unified real-time communications (URTC) in operating rooms (ORs) for image-guided interventions, where imagers, sensors, surgical robots,and computers from different vendors work cooperatively. This URTC will ensure the seamless data flow among those components and enable a closed-loop process of planning, control, delivery, and feedback. The specification of OpenIGTLink is open, and can be used without any license fee; hence OpenIGTLink is suitable for both industrial and academic developers.

Since the OpenIGTLink project stated in 2007, three major revisions have been made to the protocol. Most of the OpenIGTLink interfaces currently available comply with version 2 protocol. Please refer to the following pages for detailed specification of each version of the protocol

What’s New in Version 3

We are currently undergoing a transtion from version 2 to 3.x. At past sevral Project Weeks, a group of application developers from several institutions got together and discussed the limitations in version 2, and drafted an extended protocol, which was later released as version 3. The following changes have been made:

  • A new message structure. The body in the former protocol was splitted into extended header, content, and metadata. The message now consists of the following sections:
    • Header (58 bytes)
    • Extended Header (variable length)
    • Content (variable length)
    • Metadata (variable length)
  • The header section has the same format as version 2 protocol, and should contain the following information:
    • The header version (the first two bytes) will be incremented to ‘0x0002’
    • The Body size is the total byte size for the extended header, content, and Metadata. This will allow old clients to skip the entire message and read the sucessive message properly.
    • The other fields are filled in the same as the previous version.
  • The extended header section contains the following fields:
    • Extended header size (EXT_HEADER_SIZE) (2 bytes)
    • Metadata size (METADATA_SIZE) (2 bytes)
    • Message ID (MSG_ID) (4 bytes)
  • The content section is equivalent to the body section in the previous version.
    • The size of content can be computed as: BODY_SIZZE - (EXT_HEADER_SIZE + METADAT_SIZE)
  • The metadata section contains pairs of ‘key’ and ‘value’ strings. Keys are ASCII string, while values can be stored using different encodings.

Message Structure

The OpenIGTLink protocol defines a set of message formats for several data types that are frequently used for image-guided and robot-assisted interventions. Those formats are used to compose messages that are transferred from one device to another over the local area network. The application developers can choose their own transportation layer that fits for their application and environment (e.g. TCP, UDP, WebSocket).

While the developers are encouraged to use the data types defined in the standard protocol, they have an option to define a new message format. User-defined messages can be mixed with the standard message types, since the protocol has a mechanism to skip a message, if the receiver does not know its format.

In OpenIGTLink Version 3, each OpenIGTLink message consists of the following four sections:

  0         58 
  |  HEADER  | EXT_HEADER  |      CONTENT      | META_DATA | 
             |<----------------- Body -------------------->|
  • Header (58 bytes)
  • Extended Header (variable length) (New in Version 3)
  • Content (variable length)
  • Metadata (variable length) (New in Version 3)

The last three sections are called “Body” for the sake of convention. Since there were no Extended Header and Metadata in the older versions before to Version 3, the Body section was solely used by the message content. Therefore, the Body Size in the Header represented the size of the message content as well as the size of the rest of the message. As a result, many old applications assume that the Body Size in the Header equals the size of the message content.

When the message structure was revised for Version 3, we redefined the Body Size as the total size of Extended Header, Content, and Metadata, because this new definition allows the old OpenIGTLink programs can still tell the size of the rest of the message, and skip reading the byte stream until the beginning of the next message. The size of the content can be computed as: BODY_SIZE - (EXT_HEADER_SIZE + METADAT_SIZE).

Header + Extended Header

  0   2                       14                                      34
  | V | TYPE                  | DEVICE_NAME                           | 

  34              42              50              58
  | TIME_STAMP    | BODY_SIZE     | CRC64         |
  58                60              64        68

The formats of the Header and Extended Header sections are consistent among all message types, and can be interpreted by any software that has an OpenIGTLink interface. The Header contains device type (or data type) name, which specifies the format of the body. The Extended Header section provides a mechanism to attach application-specific meta-data to the message along with the Metadata section.


The format of contenst section is type-dependent. Please see individual type definition page linked below.


Meta-data are given in the form of an associative array (i.e. pairs of “key” and “value”) in the protocol. The Content section contains the data. The format of the Content section is defined for each data type.

Metadata header:

  0             2             4                   6               10
                |<-------------- Metadata 0 --------------------->|

      10            12                  14              18	 
  ... | KEY_SIZE_1  | VALUE_ENCODING_1  | VALUE_SIZE_1  | ...
      |<--------------- Metadata 1 -------------------->|    
      |<----------Metadata N-1 (=INDEX_COUNT-1)-------->|

Metadata body:

  +--------+---------+--------+----------+----    ----+--------+-----------+
  | KEY_0  | VALUE_0 | KEY_1  | VALUE_1  |    ...     |KEY_N-1 | VALUE_N-1 |
  +--------+---------+--------+----------+----    ----+--------+-----------+
  |<-- Metadata 0 -->|<-- Metadata 1 --->|            |<-- Metadata N-1 -->|

Please refer the following pages for the detailed format of the Header, Extended Header, and Meta-Data sections.

Message Types

Following types are supported.

Message types inherited from version 1

Message types introduced in version 2

Message types introduced in version 3

Query Mechanism

See Query Mechanism for detail.