Give options room in each packet

The data sender MUST reduce data length for the IP and TCP options it actually includes.

• The sender, not the advertising receiver, knows which optional headers it puts in each outgoing packet. Option lengths can vary between packets; MSS does not renegotiate for every such change.
• With a 1460-byte IPv4 MSS and a 1500-byte packet budget, adding 12 bytes of TCP options to 1460 data bytes would make 1512 bytes. Reducing data to 1448 makes 20 +20 +12 +1448 =1500.
• The 12-byte option-and-padding allowance is illustrative; it is not the size of the MSS option, which is 4 bytes. TCP options and padding occupy the TCP header, not application data.
• IPv6 uses a 40-byte base header instead of 20. With the same chosen 1500-byte budget, MSS 1440 and 12 bytes of additional headers leave 1428 data bytes. IPv6 extension headers also consume the per-packet budget; current terminology is clarified by RFC 9293 section 3.7.1.
• These calculations assume a suitable receive MSS and a path budget no smaller than 1500. A peer MSS alone never overrides a smaller IP-layer transmission limit.

The example packs one segment up to the chosen limit; TCP need not fill every segment. Additional bytes denote the headers actually present, including relevant padding, rather than guessed future options. No Timestamp negotiation or option wire parser is simulated.