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to add actions that run when a TCP frame is sent or received on a TCP session in the ESTABLISHED state. In the base tree, this functionality is only used for the h_ertt module, which is used by the cc_cdg, cc_chd, cc_hd, and cc_vegas congestion control modules. Presently, we incur overhead to check for hooks each time a TCP frame is sent or received on an ESTABLISHED TCP session. This change adds a new compile-time option (TCP_HHOOK) to determine whether to include the hhook(9) framework for TCP. To retain backwards compatibility, I added the TCP_HHOOK option to every configuration file that already defined "options INET". (Therefore, this patch introduces no functional change. In order to see a functional difference, you need to compile a custom kernel without the TCP_HHOOK option.) This change will allow users to easily exclude this functionality from their kernel, should they wish to do so. Note that any users who use a custom kernel configuration and use one of the congestion control modules listed above will need to add the TCP_HHOOK option to their kernel configuration. Reviewed by: rrs, lstewart, hiren (previous version), sjg (makefiles only) Sponsored by: Netflix Differential Revision: https://reviews.freebsd.org/D8185
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