The MINIMUM size of the TCP HEADER (without options) is

1. The TCP header has fixed mandatory fields totalling 20 bytes: - Source port (2 bytes), Destination port (2 bytes) - Sequence number (4 bytes), Acknowledgement number (4 bytes) - Data offset + flags + window (4 bytes) - Checksum (2 bytes), Urgent pointer (2 bytes) 2. Total: 20 bytes. With OPTIONS, the header can grow up to 60 bytes (in 4-byte multiples). 3. UDP header is 8 bytes (option A) — much smaller because UDP is connectionless. 4. IPv6 header is 40 bytes fixed (option D). _Source: RFC 1180 "A TCP/IP Tutorial", §7 (TCP) + RFC 793._