![]() ![]() Routing- The Routing field lists Source Route Entries (SREs).This is useful when carrying passenger protocols that function poorly when they receive packets out of order (such as LLC2-based protocols). Sequence Number- The two endpoints can use the sequence number to track the sequence in which packets are being received and optionally drop packets that arrive out of sequence.For the sake of this chapter, we will focus on the more common point-to-point GRE tunnels. These tunnels require that a key be defined in order for the traffic to be distinguished. An example of this is the not very widely-deployed multipoint GRE tunnels. The Key field provides this context and defines a logical traffic flow between encapsulator and decapsulator. For example, packets might need to be routed based on context information not present in the encapsulated data. This is obviously not a feature to be relied on for security, because as soon as an attacker figures out the key simply by looking at a GRE packet, he or she can generate as authentic a GRE packet as the original encapsulator.Īnother use of the key is to identify individual traffic flow within a tunnel. The key needs to be manually configured on both the endpoints. The two tunnel endpoints accept GRE packets only with the correct key in the header. In essence, the key prevents misconfiguration or injection of packets from a foreign source. ![]() This is a very weak form of security offered by GRE. ![]()
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