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https://www.ciscopress.com/articles/article.asp?p=2756480&seqNum=3#:~:text=The%20OPEN%20message%20is%20used,that%20establish%20the%20session%20capabilities.

The OPEN message is used to establish a BGP adjacency. Both sides negotiate session capabilities before a BGP peering establishes.

The OPEN message contains the BGP version number, ASN of the originating router, Hold Time, BGP Identifier, and other optional parameters that establish the session capabilities.

1. Marker (16 bytes): This field is used for synchronization and is typically set to all ones.

2. Length (2 bytes): Total length of the message, including the OPEN message header and the optional parameters.

3. Type (1 byte): Message type (OPEN message has a type value of 1).

4. Version (1 byte): BGP version number (typically 4 for BGP-4).

5. AS Number (2 bytes): The Autonomous System (AS) number of the sender.

6. Hold Time (2 bytes): The suggested hold time for the BGP session in seconds. It represents the maximum time between the receipt of successive KEEPALIVE or UPDATE messages.

7. BGP Identifier (4 bytes): A 32-bit value that uniquely identifies the sender of the OPEN message.

8. Optional Parameters Length (1 byte): Length of the Optional Parameters field.

9. Optional Parameters (variable): Additional parameters that may be included based on negotiation between the BGP peers. Examples include capabilities, authentication information, etc.