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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.
Example uses [ edit ]
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.
Example protocol stack [ edit ]
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.
IP as a delivery protocol [ edit ]
GRE packets that are encapsulated within
IP use IP protocol type 47. 
Standard GRE Packet Header [ edit ]
A standard GRE packet header structure, as defined by
RFC 2784 and RFC 2890, is represented in the diagram below.
Checksum (optional) Reserved1
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.
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.
Key Payload Length
Key Call ID
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 [ edit ]
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)
References [ edit ]
^ RFC 1702: Generic Routing Encapsulation over IPv4 networks. October 1994.
External links [ edit ]