UTT Technologies Chapter 12 VPN
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8. The PPTP receives the PPTP packets from the PPTP server, and performs
decapsulation ((15) in Figure 12-2).
9. The PPTP forwards the user data (i.e., original packets) to their intend destinations
((16) in Figure 12-2).
10. The PPTP tunnel is terminated manually by the user or automatically due to no
activity for some time ((17) in Figure 12-2).
11. After the PPTP tunnel is terminated, the PPTP
’s virtual interface returns to the
listening state ((18) in Figure 12-2).
12.1.1.3
User Authentication
PPTP provides user authentication to authenticate the user attempting the PPTP
connection by PPP-based user authentication modes such as PAP, CHAP, etc. Note that
the two endpoints of a PPTP tunnel should use the same authentication mode.
On the Router, it allows you to choose PAP, CHAP or Either as the user authentication
mode for a PPTP. It also allows you to choose None, which means that no authentication
is performed. By default, the authentication mode is Either, which means that the PPTP
will automatically negotiate it with peer.
12.1.1.4
Data Confidentiality
PPTP doesn’t provide any data encryption service by itself; it uses PPP compression and
encryption mechanisms (such as CCP, PPE, etc.) to provide data confidentiality.
12.1.1.5
MTU and Fragmentation
The Router will fragment an IP packet if it exceeds the MTU of the outbound physical
interface. For example, a standard Ethernet-type interface has a MTU of 1500 bytes, thus
the Router will fragment a packet exceeding 1500 bytes in order to transmit it over the
Ethernet interface.
With PPTP, the addition of PPTP headers may cause IP fragmentation. When an IP
packet is nearly the size of MTU of the outbound physical interface (for example, ERP or
FTP packets are often relatively large), and it is further encapsulated with PPTP headers,
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
