The ppp headers and trailers used to create a ppp frame

It should never be used as a form of weak security as simple packet sniffing will reveal its value. The key can identify an individual traffic flow within a tunnel should this facility be required.

The key field contains a four-octet number that was inserted by the encapsulator.

The checksum field contains the IP (oneвЂ™s complement) checksum sum of all the 16-bit words in the GRE header and the payload packet.

The GRE protocol header supports checksums, keys and sequencing, all of which are optional. The reconstituted datagram is then routed within the internal network and delivered to the specified station. When the peer receives this packet, it is decapsulated (the IP and GRE headers are removed), thereby exposing the original IP header. The tunnelling process creates a new IP header with a destination address that is the remote end-point of the tunnel and that is reachable over the Internet.

Usually, the destination of the original packet is not deliverable over the Internet because it is an address from a private IP range. The encapsulation process is illustrated in Figure 3 and involves the prepending of a GRE header and a fresh IP header to the original datagram. Several other tunnelling technologies also use enhanced versions of GRE as the encapsulating protocol, including the Layer II tunnelling technology PPTP (Point-to-Point Tunnelling Protocol ) that is discussed in Section 5.2.4. When employed natively, GRE is suited to static tunnels that remain configured on the two tunnel endpoints, often Internet-connected routers, regardless of whether data is flowing at any given instant. It is a transport (Layer IV) protocol operating at the same level as TCP, UDP and ICMP (Internet Control Message Protocol) and is assigned IP protocol number 47. This is a protocol developed by CiscoВ® that can encapsulate a wide variety of packet types within IP tunnels. The more obscure protocols are too specialised to warrant further mention. This guide concerns itself with the two commonest types: GRE (Generic Routing Encapsulation) tunnels and IPSec tunnelling mode, which is covered in Section 6. There are a number of types of Layer III tunnelling that are distinguished by the encapsulating protocol employed. This type of tunnelling is often associated with inter-site VPNs in which the two tunnel endpoints are both perimeter routers. Layer III TunnellingДЄn IP datagram is encapsulated and this takes place before any Layer II components (headers and trailers) have been applied. In Layer II tunnelling it is a data-link protocol (usually PPP). In a Layer III tunnel, the passenger protocol is a network-layer protocol such as IP or AppleTalkВ®. There are two types of tunnelling, distinguished by the nature of the passenger protocol. the carrier protocol is used by the network carrying the tunnelled packet and must be IP if tunnelling is to take place over the Internet.the encapsulating protocol is the transport-layer protocol that is wrapped around the original data.the passenger protocol is the original packet and will be either a network layer protocol (IP, IPX, AppleTalkВ® etc.) or a PPP frame.Three protocols are involved in the process: When a tunnelled packet is received by the destination end point, the headers concerned with the tunnel are stripped away to reveal the original packet that is delivered to the final destination.ДЄll tunnelling involves encapsulation of the original packet or frame before it is released onto the Internet. A VPN consists of two stations, known as the tunnel endpoints, that perform the necessary operations. The solution is to use tunnelling, in which a packet destined for the remote site is placed inside another IP packet with globally routable source and destination addresses.

Yet another reason for tunnelling is an encryption of the original packet: if all its fields including the IP header ones are encrypted then routers just canвЂ™t do their job. However the purpose of a VPN is to allow a remote host or site to become part of the LAN, and so the security measures used to guard against intrusion from the Internet must be selectively circumvented to allow the VPN to work. Either or both of these measures may be present at both ends of a VPN, preventing external packets from reaching systems connected to the LAN.

Many corporate networks are shielded from the outside world by firewall devices or by the simple expedient of running the network on private IP addresses that are not routed over the global Internet. Multi-site Connectivity Advisory Service.

The ppp headers and trailers used to create a ppp frame