The following figure shows the packet format for the Enhanced Authentication Option.
Option Syntax:
Kind: 8 bits
The Kind field identifies the TCP Enhanced Authentication Option. This value will be assigned by IANA.
Length: 8 bits
The Length field specifies the length of the TCP Enhanced Authentication Option, in octets. This count includes two octets representing the Kind and Length fields.
The valid range for this field is from 4 to 40 octets, inclusive.
For all algorithms specified in this memo the value will be 16 octets.
T-Bit: 1 bit
The T-bit specifies whether TCP Options were omitted from the TCP header for the purpose of MAC calculation. A value of 1 indicates that all TCP options other than the Extended Authentication Option were omitted. A value of 0 indicates that TCP options were included.
The default value is 0.
K-Bit: 1 bit
This bit is reserved for future enhancement. Its value MUST be equal to zero.
Alg ID: 6 bits
The Alg ID field identifies the MAC algorithm.
Res: 2 bits
These bits are reserved. They MUST be set to zero.
Key ID: 6 bits
The Key ID field identifies the key that was used to generate the message digest.
Authentication Data: Variable length
The Authentication Data field contains data that is used to authenticate the TCP segment. This data includes, but need not be restricted to, a MAC. The length and format of the Authentication Data Field can be derived from the Alg ID.
The Authentication for TCP-based Routing and Management Protocols draft provides and overview of the TCP Enhanced Authentication Option. The details of this feature are described in draft-bonica-tcp-auth-04.txt.