RFC 793
>IP Packet Structure An IP packet consists of a variable-length header that prefixes the IP data. The information contained in the header is outlined in Table 2.5.
Uses four bits to indicate the number of 32-bit words in the IP header. Minimum header size is 20 bytes; the smallest header length is 0x5. IP options can extend the minimum IP header size by four bytes at a time. If an IP option does not use all four bytes of the Option field, remaining bits are completed with zeroes so the IP header is 32 bits (four bytes).<
microsoft
altavista.com search
+"TCP packet" +"header size"