Managers Use Special Packets Called MADs

Request MAD

The various managers use special packets called Management Datagrams (MADs) to request that an operation (i.e., a method) be performed on a device attribute. A request MAD has the following basic characteristics:

• The MAD message is wholly contained in a single packet's data payload field and the data payload field always contains exactly 256 bytes.

• Some of the elements contained in the MAD are:

  - Management Class: Identifies the manager that issued the packet and therefore the management agent within the device that is to process the request MAD.
  
  

  - Method: Specifies the type of operation the target management agent is to perform on the specified attribute. As an example, a Get method performs an attribute read, while a Set method performs an attribute write operation.
  
  

  - Attribute ID: Specifies the attribute to be acted upon (e.g., to read or written).
  
  

  - Attribute Modifier: Attribute-specific; not required for many attribute/method combinations. Specifies additional information regarding the targeted attribute. As an example, if the manager is targeting the PortInfo attribute on a CA, router, or switch, the Modifier specifies the target port number.
  
  

  - Data area: Content depends on the Method and Attribute. Some examples would be:
  
  

  - If the Method is a Set operation, the data area contains the data to be written to the specified attribute.

  - If the Method is a Get operation, the data area contents is undefined in the request MAD, but contains the requested attribute's contents in the corresponding response MAD returned by the device.

Response MAD

Upon receipt of a request MAD from its respective manager, the MA performs the operation on the indicated attribute and returns the results in a response MAD. The response MAD contains (among other things):

• Status field: This 16-bit field indicates the operation's completion status. The Status field is only valid in a response MAD generated by the target in response to a request MAD. It is formatted as follows:

  - Lower 8 bits = status common to all classes. These are:

  - Busy, MAD discarded (not an error).

  - Redirect required (not an error).

  - Class not supported.

  - Method not supported.

  - Method/Attribute combination not supported.

  - One or more Attribute fields contain bad values.

  - Upper 8 bits = class-specific status.

• Data field. The content of this field is defined by the type of request MAD this is a response to. Some examples would be:

  - If the request was an attribute read (i.e., a Get), the data field contains the data read from the attribute indicated in the request MAD.

  - If the request was an attribute write (i.e., a Set), the data field contains the data read from the attribute indicated in the request MAD after the Set operation (i.e., the attribute write) was performed.