MTU & MSS

Maximum Transmission Unit & Maximum Segment Size

                    <--------------------------- IP MTU ------------------->
                    <--------------------- Ethernet MTU ------------------->
                                               <--------- TCP MSS --------->
       +------------+------------+-------------+---------------------------+------------+
FRAME  | Eth-Header |  IP-Header |  TCP-Header |        Data payload       | Eth-trailer|
       +------------+------------+-------------+---------------------------+------------+
BYTES        14           20            20                 1460                  4

Packet Unit Size

Packet Unit	Minimum Size	Maximum Size	Reasons
IPv4 packet	20 Bytes	65535 Bytes	16-bit Total Length field in an IPv4 header can store values from 0 to 65535. As IP header is 20 bytes which defines the minimal.
Ethernet frame	64 Bytes	1518 Bytes	When encapsulated in Ethernet, the IP packet has a minimum 46-byte payload (if an 802.1Q tag is absent). Ethernet MTU is 1500 bytes (or 1522 with VLAN tagging).