Generic Routing Encapsulation
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
- 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
| OSI model layer | Protocol |
|---|---|
| 5. Session | X.225 |
| 4. Transport | UDP |
| 3. Network (GRE-encapsulated) | IPv6 |
| Encapsulation | GRE |
| 3. Network | IPv4 |
| 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.
IP as a delivery protocol
GRE packets that are encapsulated within IP use IP protocol type 47.[1]
Packet header
Standard GRE Packet Header
A standard GRE packet header structure, as defined by RFC 2784 and RFC 2890, is represented in the diagram below.
| Bits 0–3 | 4–12 | 13–15 | 16–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | K | S | Reserved0 | Version | Protocol Type | ||||||||||||||||||||||||||
| Checksum (optional) | Reserved1 (optional) | ||||||||||||||||||||||||||||||
| Key (optional) | |||||||||||||||||||||||||||||||
| Sequence Number (optional) | |||||||||||||||||||||||||||||||
- 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.
PPTP GRE Packet Header
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.
| Bits 0–4 | 5–7 | 8 | 9-12 | 13–15 | 16–31 | ||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | R | K | S | s | Recur | A | Flags | Version | Protocol Type | ||||||||||||||||||||||
| Key Payload Length | Key Call ID | ||||||||||||||||||||||||||||||
| Sequence Number (optional) | |||||||||||||||||||||||||||||||
| Acknowledgement Number (optional) | |||||||||||||||||||||||||||||||
- 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)
References
External links
- Generic Routing Encapsulation, Subprotocol homepage at Cisco
- Generic Routing Encapsulation, Entry in Cisco DocWiki (formerly known as the "Internetworking Technology Handbook")
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