H3C Technologies H3C SecPath F1000-E - Manual (Page 3)

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Format of an encapsulated packet
Figure 2 shows the format of an encapsulated packet.
Figure 2 Format of an encapsulated packet
As an example,
Figure 3 shows the format of an X packet encapsulated for transmission over an IP tunnel.
Figure 3 Format of an X packet encapsulated for transmission over an IP tunnel
These are the terms involved:
• Payload: Packet that needs to be encapsulated and transmitted.
• Passenger protocol: Protocol that the payload packet uses, IPX in the example.
• Encapsulation or carrier protocol: Protocol used to encapsulate the payload packet, that is, GRE.
• Delivery or transport protocol: Protocol used to encapsulate the GRE packet and then forward the
packet to the other end of the tunnel, IP in this example.
Depending on the transport protocol, two tunnel modes are present: GRE over IPv4 and GRE over IPv6.
De-encapsulation process
De-encapsulation is the reverse process of encapsulation:
1. Upon receiving an IP packet from the tunnel interface, Device B checks the destination address.
2. If the destination is itself, Device B strips off the IP header of the packet and submits the resulting
packet to the GRE protocol.
3. The GRE protocol checks the key, checksum and sequence number in the packet, and then strips
off the GRE header and submits the payload to the X protocol for forwarding.
NOTE:
Encapsulation and de-encapsulation processes on both ends of the GRE tunnel and the resulting increase
in data volumes will degrade the forwarding efficiency for the GRE-enabled device to some extent.