Generic Routing Encapsulation
| Communication protocol | |
| Purpose | Network tunneling |
|---|---|
| Developer(s) | Cisco Systems |
| Introduction | 1994[1] |
| RFC(s) | RFC1701, RFC1702, RFC2784 |
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]
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) | 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.
IP as a delivery protocol[]
GRE packets that are encapsulated within IP use IP protocol type 47.[3]
Packet header[]
Extended GRE packet header (RFC 2890)[]
The extended version of the GRE packet header as defined by RFC 2890.
| Bits 0–3 | 4-12 | 13-15 | 16–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | K | S | Reserved0 | Version | Protocol Type | ||||||||||||||||||||||||||
| Checksum (optional) | Reserved 1 (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.
Standard GRE packet header (RFC 2784)[]
A standard GRE packet header structure, as defined by RFC 2784, is represented in the diagram below.
| 0 | Bits 1–12 | 13–15 | 16–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Reserved0 | Version | Protocol Type | ||||||||||||||||||||||||||||
| Checksum (optional) | Reserved1 (optional) | ||||||||||||||||||||||||||||||
- C
- Checksum bit. Set to 1 if a checksum 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.
Standard GRE packet header (RFC 1701, obsolete)[]
The newer structure superseded the original structure as defined in RFC 1701:
| Bits 0–4 | 5–7 | 8-12 | 13–15 | 16–31 | |||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | R | K | S | s | Recur | Flags | Version | Protocol Type | |||||||||||||||||||||||
| Checksum (optional) | Offset (optional) | ||||||||||||||||||||||||||||||
| Key (optional) | |||||||||||||||||||||||||||||||
| Sequence Number (optional) | |||||||||||||||||||||||||||||||
| Routing (optional, variable length) | |||||||||||||||||||||||||||||||
The original GRE RFC defined further fields in the packet header which became obsolete in the current standard:
- R
- Routing Bit. Set to 1 if Routing and Offset information are 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.
- s
- Strict source route bit.
- Recur
- Recursion control bits.
- Flags
- Reserved for future use, set to 0.
- Offset
- 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
- 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.
- Routing
- 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), 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
- Acknowledgment number present. Set to 1 if an acknowledgment number is supplied, indicating a PPTP GRE acknowledgment 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)
- RFC 8086: GRE-in-UDP Encapsulation (proposed standard)
See also[]
- Network Virtualization using Generic Routing Encapsulation - carries L2 packets over GRE
References[]
- ^ Generic Routing Encapsulation (GRE). IETF. October 1994. doi:10.17487/RFC1701. RFC 1701. Retrieved May 14, 2018.
- ^ [1], "Generic routing encapsulation tunnel keepalives", issued 2002-07-01
- ^ RFC 1702: Generic Routing Encapsulation over IPv4 networks. October 1994.
External links[]
 |
Wikimedia Commons has media related to Category:Generic Routing Encapsulation. |
- Generic Routing Encapsulation, Subprotocol homepage at Cisco
- Generic Routing Encapsulation, Entry in Cisco DocWiki (formerly known as the "Internetworking Technology Handbook")
Categories:
- Tunneling protocols