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BGP Message Generation and Transport, and General Message Format
(Page 3 of 3)
The Marker Field, Message Synchronization and Authentication
The Marker field is the most
interesting one in the BGP message format, and as mentioned in the table
is used for both synchronization and authentication. BGP uses a single
TCP session to send many messages in a row. TCP is a stream-oriented
transport protocol which just sends bytes across the link without any
knowledge of what the bytes represent. This means that the
protocol using TCP is responsible for deciding where the line is drawn
between data units, in this case BGP messages.
Normally, the Length field
tells each BGP device where to draw the line between the end of one
message and the start of the next. However, it is possible that due
to various conditions a device might lose track of where the message
boundary is. The Marker field is filled with a recognizable pattern
that clearly marks the start of each message, to keep BGP peers synchronized.
Before a BGP connection is established,
the Marker field is just filled with all ones. Thus, this is
the pattern used for Open messages. Once a BGP session is negotiated,
if agreement is reached on an authentication method between the two
devices, the Marker field takes on the additional role of authentication.
Instead of looking for a Marker containing all ones, BGP devices
look for a pattern generated using the agreed-upon authentication method.
Detection of this pattern simultaneously synchronizes the devices and
ensures that messages are authentic.
In extreme cases, BGP peers may be
unable to maintain synchronization, and if so, a Notification
message is generated and the session is closed. This will also happen
if the Marker field contains the wrong data when authentication
is enabled.
 Key Concept: All four BGP message types use a general message format that contains three fixed header fields—Marker, Length and Type—and room for a message body that differs for each message type. The large Marker field is used to denote the start of a new BGP message, and is also used to facilitate the BGP authentication method.
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