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Generic Routing Encapsulation (GRE) carries one network-layer packet inside another. An outer delivery header moves the GRE packet across a network, while the GRE header identifies the type of packet inside it. GRE does not itself encrypt traffic or guarantee delivery.
What does GRE stand for?
GRE stands for Generic Routing Encapsulation. It is a protocol mechanism for encapsulating a packet of one network-layer protocol inside another network-layer protocol. RFC 2784 describes it as “a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol.”
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In simpler terms, GRE wraps an original packet so another network-layer protocol can carry it. The standards define how that wrapping works; understanding GRE does not require a particular consumer product.
How is a GRE packet arranged?
A basic GRE packet has three parts, in order:
- Delivery header: The outer protocol header carries the GRE packet toward its destination.
- GRE header: Its protocol-type field identifies the kind of packet being carried.
- Payload: The original packet being encapsulated.
RFC 2784’s base GRE header includes a checksum-present flag, reserved bits, a version field, and the protocol type. The checksum and reserved fields are present when the checksum-present bit is set.
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What do the optional GRE Key and Sequence Number do?
RFC 2890 defines optional extensions to the GRE header. They are not fields that should be assumed to appear in every GRE packet.
Key
The four-octet Key field can identify a traffic flow within a tunnel and provide context for handling packets. Despite its name, it is not a cryptographic key and does not secure the traffic.
Sequence Number
The four-octet Sequence Number can help a receiver establish the order in which packets were transmitted. RFC 2890 describes the intended behavior as unreliable but in-order delivery: the field supports ordering, not guaranteed delivery.
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No. GRE provides encapsulation, not encryption or a guarantee that packets will arrive. If GRE traffic and its tunneled payload need protection, RFC 2890 discusses protecting them with IPsec ESP or AH. The optional GRE Key does not provide that protection.
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GRE standards
- RFC 2784, “Generic Routing Encapsulation (GRE)”, published by the RFC Editor in March 2000, specifies the GRE packet model and base header.
- RFC 2890, “Key and Sequence Number Extensions to GRE”, published by the RFC Editor in September 2000, specifies the optional Key and Sequence Number fields.
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