1. 以太幀
//包括二層以太頭和FCS：
Ethernet MinSize = 512bit = 64 Byte
Ethernet MaxSize = 1518 Byte
2. IP MTU
Ethernet MTU = 1518 - 18 ( 6 SRCMAC+ 6 DSTMAC+ 2 TYPE+ 4 CRC) = 1500 B
IP MTU <= Ethernet MTU
3. MSS
TCP MSS = 1500 - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1460 B
4. UDP
UDP MTU = 1500 - 28 ( 20 IP_HEADER + 8 UDP_HEADER ) = 1472 B
5. PPPoE
PPPoE MTU/MRU = 1500 - 8 ( 6 PPPoE_SESSION + 2 PPP_HEADER ) = 1492 B
6. TCP over PPPoE
TCP over PPPoE MSS = 1492 ( PPPoE MTU/MRU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1452
7. PPTP MTU
PPTP MTU = 1500 - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1444 B
8. TCP over PPTP
TCP over PPTP MSS = 1444 ( PPTP MTU/MRU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1404
9. L2TP MTU
L2TP MTU = 1500 - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1460 B
10. TCP over L2TP
TCP over L2TP MSS = 1460 ( L2TP MTU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1420 B
11. PPTP over PPPoE
PPTP over PPPoE MTU/MRU = 1492 ( PPPoE MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1436 B
12. PPTP over PPTP
PPTP over PPTP MTU/MRU = 1444 ( PPTP MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1388 B
13. PPTP over L2TP
PPTP over L2TP MTU/MRU = 1460 ( L2TP MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1404 B
14. L2TP over PPPoE
L2TP over PPPoE MTU/MRU = 1492 ( PPPoE MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1452 B
15. L2TP over PPTP
L2TP over PPTP MTU/MRU = 1444 ( PPTP MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1404 B
16. L2TP over L2TP
L2TP over L2TP MTU/MRU = 1460 ( L2TP MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1420 B
通過(guò)上面計(jì)算炉擅，PPTP over PPPoE的準(zhǔn)確MTU值為1436吉懊，L2TP over PPPoE的準(zhǔn)確MTU值為1452碧绞。
MSS = MTU - 40
