Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network.
In conjunction with PPTP to create VPNs.
In conjunction with IPsec VPNs to allow passing of routing information between connected networks.
In mobility protocols.
In A8/A10 interfaces to encapsulate IP data to/from Packet Control Function (PCF).
Linux and BSD can establish ad-hoc IP over GRE tunnels which are interoperable with Cisco equipment.
Distributed denial of service (DDoS) protected appliance to an unprotected endpoint.
Based on the principles of protocol layering in OSI, protocol encapsulation, not specifically GRE, breaks the layering order. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.
GRE packets that are encapsulated within IP use IP protocol type 47.
Standard GRE Packet Header
A standard GRE packet header structure, as defined by RFC 2784 and RFC 2890, is represented in the diagram below.
C
Checksum bit. Set to 1 if a checksum is present.
K
Key bit. Set to 1 if a key is present.
S
Sequence number bit. Set to 1 if a sequence number is present.
Reserved0
Reserved bits; set to 0.
Version
GRE Version number; set to 0.
Protocol Type
Indicates the ether protocol type of the encapsulated payload. (For IPv4, this would be hex 0800.)
Checksum
Present if the C bit is set; contains the checksum for the GRE header and payload.
Reserved1
Present if the C bit is set; is set to 0.
Key
Present if the K bit is set; contains an application-specific key value.
Sequence Number
Present if the S bit is set; contains a sequence number for the GRE packet.
The Point-to-Point Tunneling Protocol (PPTP), defined in RFC 2637, uses a variant GRE packet header structure, represented below. PPTP creates a GRE tunnel through which the PPTP GRE packets are sent.
C
Checksum bit. For PPTP GRE packets, this is set to 0.
R
Routing bit. For PPTP GRE packets, this is set to 0.
K
Key bit. For PPTP GRE packets, this is set to 1. (All PPTP GRE packets carry a key.)
S
Sequence number bit. Set to 1 if a sequence number is supplied, indicating a PPTP GRE data packet.
s
Strict source route bit. For PPTP GRE packets, this is set to 0.
Recur
Recursion control bits. For PPTP GRE packets, these are set to 0.
A
Acknowledgement number present. Set to 1 if an acknowledgement number is supplied, indicating a PPTP GRE acknowledgement packet.
Flags
Flag bits. For PPTP GRE packets, these are set to 0.
Version
GRE Version number. For PPTP GRE packets, this is set to 1.
Protocol Type
For PPTP GRE packets, this is set to hex 880B.
Key Payload Length
Contains the size of the payload, not including the GRE header.
Key Call ID
Contains the Peer's Call ID for the session to which the packet belongs.
Sequence Number
Present if the S bit is set; contains the GRE payload sequence number.
Acknowledgement Number
Present if the A bit is set; contains the sequence number of the highest GRE payload packet received by the sender.
Standards
RFC 1701: Generic Routing Encapsulation (GRE) (informational)
RFC 1702: Generic Routing Encapsulation over IPv4 networks (informational)
RFC 2637: Point to Point Tunneling Protocol (informational)
RFC 2784: Generic Routing Encapsulation (GRE) (proposed standard, updated by RFC 2890)
RFC 2890: Key and Sequence Number Extensions to GRE (proposed standard)
