8.2 BGP Basic Operations
8.2.3 BGP message header format
The BGP message header format is a 16-byte marker field, followed by a 2-byte length field and a 1-byte type field. The figure illustrates the basic format of the BGP message header.

There may or may not be a data portion following the header, depending on the message type. KEEPALIVE messages, for example, consist of the message header only, with no following data.

The marker field is used to either authenticate incoming BGP messages or to detect loss of synchronization between two BGP peers. The marker field can have two formats:

  • If the type of the message is OPEN or if the OPEN message has no authentication information, the marker field must be all ones.
  • Otherwise, the marker field will be computed based on part of the authentication mechanism used.

The length indicates the total BGP message length, including the header. The smallest BGP message is no less than 19 bytes (16 + 2 + 1) and no greater than 4096.

The type indicates the message type, from the following possibilities:

  • OPEN
  • UPDATE
  • NOTIFICATION
  • KEEPALIVE

The following sections examine the purpose and format of each of the four message types in more detail.