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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 or point-to-multipoint links over an Internet Protocol network.[2]
| Communication protocol | |
| Purpose | Network tunneling |
|---|---|
| Developer(s) | Cisco Systems |
| Introduction | 1994[1] |
| RFC(s) | 1701, 1702, 2784 |
- In conjunction with PPTP to create VPNs.
- In conjunction with IPsec VPNs to allow passing of routing information between connected networks.
- In mobility management 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
| OSI model layer | Protocol example |
|---|---|
| 7. Application | HTTP |
| 4. Transport | TCP |
| 3. Network (GRE-encapsulated) | IPv4 |
| Encapsulation | GRE |
| 3. Network | IPv6 |
| 2. Data link | Ethernet |
| 1. Physical | Ethernet physical layer |
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 directly, use IP protocol type 47 in the IPv4 header's Protocol field[3] or the IPv6 header's Next Header field.[4]
For performance reasons, GRE can also be encapsulated in UDP packets.[5] Better throughput may be achieved by using Equal-cost multi-path routing.
Extended GRE packet header (RFC 2890)
The extended version of the GRE packet header[6] is represented below:
- C (1 bit)
- Checksum bit. Set to 1 if a checksum is present.
- K (1 bit)
- Key bit. Set to 1 if a key is present.
- S (1 bit)
- Sequence number bit. Set to 1 if a sequence number is present.
- Reserved 0 (9 bits)
- Reserved bits; set to 0.
- Version (3 bits)
- GRE Version number; set to 0.
- Protocol Type (16 bits)
- Indicates the ether protocol type of the encapsulated payload. (For IPv4, this would be hex 0800.)
- Checksum (16 bits)
- Present if the C bit is set; contains the checksum for the GRE header and payload.
- Reserved 1 (16 bits)
- Present if the C bit is set; is set to 0.
- Key (32 bits)
- Present if the K bit is set; contains an application-specific key value.
- Sequence Number (32 bits)
- Present if the S bit is set; contains a sequence number for the GRE packet.
Standard GRE packet header (RFC 2784)
A standard GRE packet header structure[7] is represented in the diagram below.
- C (1 bit)
- Checksum bit. Set to 1 if a checksum is present.
- Reserved 0 (12 bits)
- Reserved bits; set to 0.
- Version (3 bits)
- GRE Version number; set to 0.
- Protocol Type (16 bits)
- Indicates the ether protocol type of the encapsulated payload. (For IPv4, this would be hexadecimal 0x0800; for IPv6, it would be 0x86DD.[4])
- Checksum (16 bits)
- Present if the C bit is set; contains the checksum for the GRE header and payload.
- Reserved 1 (16 bits)
- Present if the C bit is set; its contents is set to 0.
Original GRE packet header (RFC 1701)
The newer structure superseded the original structure:[1]
| Offsets | Octet | 0 | 1 | 2 | 3 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Octet | Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
| 0 | 0 | C | R | K | S | s | Recur | Flags | Version | Protocol Type | |||||||||||||||||||||||
| 4 | 32 | Checksum (optional) | Offset (optional) | ||||||||||||||||||||||||||||||
| 8 | 64 | Key (optional) | |||||||||||||||||||||||||||||||
| 12 | 96 | Sequence Number (optional) | |||||||||||||||||||||||||||||||
| 16 | 128 | Routing (optional, variable length) | |||||||||||||||||||||||||||||||
The original GRE RFC defined further fields in the packet header which became obsolete in the current standard:
- C (1 bit)
- Checksum bit. Set to 1 if a checksum is present.
- R (1 bit)
- Routing Bit. Set to 1 if Routing and Offset information are present.
- K (1 bit)
- Key bit. Set to 1 if a key is present.
- S (1 bit)
- Sequence number bit. Set to 1 if a sequence number is present.
- s (1 bit)
- Strict source route bit.
- Recur (3 bits)
- Recursion control bits.
- Flags (5 bits)
- Reserved for future use, set to 0.
- Version (3 bits)
- Set to 0.
- Protocol Type (16 bits)
- Indicates the ether protocol type of the encapsulated payload.
- Checksum (16 bits)
- Present if the C bit is set; contains the checksum for the GRE header and payload.
- Offset (16 bits)
- Present if R bit or C bit is set; contains valid information, only if R bit is set. An offset field indicating the offset within the Routing field to the active source route entry.
- Key (32 bits)
- Present if the K bit is set; contains an application-specific key value.
- Sequence Number (32 bits)
- Present if the S bit is set; contains a sequence number for the GRE packet.
- Routing (variable)
- Present if R bit is set; contains a list of source route entries, therefore is of variable length.
PPTP GRE packet header
The Point-to-Point Tunneling Protocol (PPTP) [8] uses a variant GRE packet header structure, represented below. PPTP creates a GRE tunnel through which the PPTP GRE packets are sent.
- C (1 bit)
- Checksum bit. For PPTP GRE packets, this is set to 0.
- R (1 bit)
- Routing bit. For PPTP GRE packets, this is set to 0.
- K (1 bit)
- Key bit. For PPTP GRE packets, this is set to 1. (All PPTP GRE packets carry a key.)
- S (1 bit)
- Sequence number bit. Set to 1 if a sequence number is supplied, indicating a PPTP GRE data packet.
- s (1 bit)
- Strict source route bit. For PPTP GRE packets, this is set to 0.
- Recur (3 bits)
- Recursion control bits. For PPTP GRE packets, these are set to 0.
- A (1 bit)
- Acknowledgment number present. Set to 1 if an acknowledgment number is supplied, indicating a PPTP GRE acknowledgment packet.
- Flags (4 bits)
- Flag bits. For PPTP GRE packets, these are set to 0.
- Version (3 bits)
- GRE Version number. For PPTP GRE packets, this is set to 1.
- Protocol Type (16 bits)
- For PPTP GRE packets, this is set to hex 880B.
- Key Payload Length (16 bits)
- Contains the size of the payload, not including the GRE header.
- Key Call ID (16 bits)
- Contains the Peer's Call ID for the session to which the packet belongs.
- Sequence Number (32 bits)
- Present if the S bit is set; contains the GRE payload sequence number.
- Acknowledgement Number (32 bits)
- Present if the A bit is set; contains the sequence number of the highest GRE payload packet received by the sender.
- 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)
- RFC 8086: GRE-in-UDP Encapsulation (proposed standard)
- Network Virtualization using Generic Routing Encapsulation - carries L2 packets over GRE
- GPRS Tunnelling Protocol - GTP-U is similar to GRE and used in cellular networks
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