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Chapter 10 VPN
the encapsulated packet is likely to exceed the MTU of the outbound physical interface.
This causes the encapsulated packet to be fragmented before transmission, and the
PPTP receiver is responsible for reassembling the fragments back into the original
encapsulated packet before decapsulation. More specifically, the receiver cannot perform
reassembly until the last fragment is received; and if one fragment is lost, the entire
original encapsulated packet must be resent, and it will also be fragmented.
Data fragmentation and reassembly can seriously degrade the system performance, so it
is highly necessary to avoid fragmentation and reassembly in the PPTP switching path. To
solve this problem, PPTP allows the client and server to negotiate PPP MRU/MTU during
PPTP tunnel establishment.
In addition, on the Gigabit Router, you can adjust the global PPTP tunnel MTU (i.e.,
tunnelmtu) to minimize the fragmentation: if an IP packet exceeds the specified MTU, it
will be fragmented by the original computer before transmission. The following two
examples describe how to calculate PPTP tunnel MTU. Figure 9-3 illustrates the format of
the PPTP packet to be sent over a static IP or DHCP Internet connection; and Figure 9-4
illustrates the format of the PPTP packet to be sent over a PPPoE Internet connection.
Therein, the sizes of standard Ethernet MTU and each encapsulation header are as
follows:
Ethernet MTU 1500 Bytes
IP Header 20 Bytes
GRE Header 8 Bytes
PPTP Header 30 Bytes (at most)
PPPoE Header 8 Bytes
Figure 9-3 PPTP Packet Format - Static IP/DHCP Internet Connection
Figure 9-4 PPTP Packet Format - PPPoE Internet Connection
Therefore, to avoid fragmentation and reassembly in the PPTP switching path, the PPTP
tunnel MTU should be smaller or equal to 1442 bytes (1500-20-8-30=1442) when the
PPTP packets are sent over a static IP or DHCP Internet connection (see Figure 9-3); and
it must be smaller or equal to 1434 bytes (1442-8=1434) when the PPTP packets are sent
over a PPPoE Internet connection (see Figure 9-4).
On the Gigabit Router, the PPTP tunnel MTU is 1400 bytes by default. In most cases,
please leave the default value because it can meet most application needs.
