From: TCP/IP Illustrated, Volume 1: The Protocols.
IHL
The Internet Header Length (IHL) field is the number of 32-bit words in the IPv4 header, including any options. Because this is also a 4-bit field, the IPv4 header is limited to a maximum of fifteen 32-bit words or 60 bytes.
Total Length
The Total Length field is the total length of the IPv4 datagram in bytes.
Protocol
The Protocol field in the IPv4 header contains a number indicating the type of data found in the payload portion of the datagram. The most common values are 17 (for UDP) and 6 (for TCP).