BGP Message Generation and Transport, and General Message Format
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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.
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