PPTP/L2TP over PPPoE的準確MTU/MRU值

1 Ethernet MinSize = 512bit = 64 Byte
2Ethernet MaxSize = 1518 Byte
3so Ethernet IP MTU = 1518 - 18 ( 6 SRCMAC+ 6 DSTMAC+ 2 TYPE+ 4 CRC) = 1500 B
4so Ethernet IP TCP MSS = 1500 - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1460 B
5so Ethernet IP UDP MTU/MRU = 1500 - 28 ( 20 IP_HEADER + 8 UDP_HEADER ) = 1472 B
6so PPPoE MTU/MRU = 1500 - 8 ( 6 PPPoE_SESSION + 2 PPP_HEADER ) = 1492 B
7so TCP over PPPoE MSS = 1492 ( PPPoE MTU/MRU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1452
8so PPTP MTU/MRU = 1500 - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1444 B
9so TCP over PPTP MSS = 1444 ( PPTP MTU/MRU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1404
10so L2TP MTU/MRU = 1500 - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1460 B
11so TCP over L2TP MSS = 1460 ( L2TP MTU/MRU ) - 40 ( 20 IP_HEADER + 20 TCP_HEADER) = 1420 B
12so PPTP over PPPoE MTU/MRU = 1492 ( PPPoE MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1436 B
13so PPTP over PPTP MTU/MRU = 1444 ( PPTP MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1388 B
14so PPTP over L2TP MTU/MRU = 1460 ( L2TP MTU/MRU ) - 56 ( 20 IP_HEADER + 20 TCP_HEADER + 12 GRE_HEADER + 4 PPP_HEADER ) = 1404 B
15so L2TP over PPPoE MTU/MRU = 1492 ( PPPoE MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1452 B
16so L2TP over PPTP MTU/MRU = 1444 ( PPTP MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1404 B
17so L2TP over L2TP MTU/MRU = 1460 ( L2TP MTU/MRU ) - 40 ( 20 IP_HEADER +8 UDP_HEADER + 8 L2TP_HEADER + 4 PPP_HEADER ) = 1420 B

故而，PPTP over PPPoE的準確MTU值是 1436，L2TP over PPPoE的準確MTU是1452。

別人說的 1400 偏保守了點。

如果你用 ADSL 上網，然後用 PPTP 來翻Wall，那麼，你實際上是 PPP 協議跑在 TCP 協議上再跑在 IP 協議上再跑在 PPP 協議上再跑在 IP 協議上再跑在以太網協議上。1518 字節的最大以太網 frame，扣來扣去，就剩下 1436。同理L2TP跑在PPP鏈路上，扣來扣去也就只剩1452B了，難怪筆者之前VPN上不去網，原來是全把數據包丟掉了。

如果讀者看明白了上面的解釋，那麼考慮你用 pptp 連公司的 vpn，公司又 pppoe(adsl撥號)，然後你再 pptp 來翻功夫網，那麼，你的 MTU 只能設為 1518-18-8-56-56=1380 字節。很簡單吧？

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    PPTP/L2TP over PPPoE的準確MTU/MRU值 – rock funs

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    PPTP/L2TP over PPPoE的準確MTU/MRU值 – rock funs