Avoid subtracting the same options twice

Do not pre-subtract possible options from MSS and then subtract them again at the sender.

• RFC 6691 section 4 compares four combinations. The recommended combination is to leave possible options out of the advertised-value calculation, then adjust the sender’s actual packet data for its headers.
• For the chosen 1500-byte IPv4 example with 12 actual option bytes: advertise 1460 and send 1448 data bytes. The IP packet is exactly 1500 bytes.
• If the receiver advertises 1448 after pre-subtracting 12, and the sender also subtracts 12, only 1436 data bytes are sent. Total length is 1488: double accounting wastes 12 bytes of this example’s budget.
• If the receiver pre-subtracts 12 and the sender makes no adjustment, this one packet happens to fit. That accidental result does not support a general rule: the receiver cannot know every outgoing packet’s option combination.
• If neither side accounts for options,1460 data bytes plus 40 fixed-header bytes plus 12 option bytes total 1512. RFC 6691 sections 3.1 and 3.2 correct earlier RFC 879/RFC 2385 advice; this guide does not recommend deploying TCP-MD5.

A simplified comparison uses the same receive and path budget and an exactly 12-byte extra-header fixture in every row. “Too short” describes unused space, not an invalid TCP packet or a mandatory minimum payload. Appropriate packet sizing does not itself authenticate TCP.