Generic routing encapsulation
{{Short description|Cisco tunneling protocol}}
{{Infobox networking protocol
| title = Generic routing encapsulation
| logo =
| logo alt =
| image =
| image alt =
| caption =
| is stack = no
| purpose = Network tunneling
| developer = Cisco Systems
| date = 1994{{Ref RFC|1701}}
| based on =
| influenced =
| osilayer =
| ports =
| rfcs = {{IETF RFC|1701|1702|2784|plainlink=yes}}
| hardware =
}}
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.{{Cite patent|title=Generic routing encapsulation tunnel keepalives|country=US|number=7801021B1|invent1=Nikolaos Triantafillis|invent2=Robert J. Ordemann|invent3=Simon D. Barber|assign1=Cisco Technology Inc.|gdate=2010-09-21|fdate=2002-07-01|url=https://patents.google.com/patent/US7801021B1/en}}
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 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=
| class="wikitable"
!OSI model layer |
| 7. Application
|HTTP |
| 4. Transport
|TCP |
| 3. Network (GRE-encapsulated)
|IPv4 |
| Encapsulation
|GRE |
| 3. Network
|IPv6 |
| 2. Data link |
| 1. Physical |
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.
Delivery protocols
GRE packets that are encapsulated within IP directly, use IP protocol type 47 in the IPv4 header's Protocol field{{Ref RFC|1702}} or the IPv6 header's Next Header field.{{Ref RFC|7676}}
For performance reasons, GRE can also be encapsulated in UDP packets.{{Ref RFC|8086}} Better throughput may be achieved by using Equal-cost multi-path routing.
Packet header
=Extended GRE packet header (RFC 2890)=
The extended version of the GRE packet header{{Ref RFC|2890}} is represented below:
:
| class="wikitable" style="text-align: center"
|+Extended GRE header format |
| style="border-bottom:none; border-right:none;"|Offsets
!style="border-left:none;"|Octet !colspan="8"|0 !colspan="8"|1 !colspan="8"|2 !colspan="8"|3 |
|---|
| style="border-top: none"|Octet
!Bit !style="width:2.75%;"|0 !style="width:2.75%;"|1 !style="width:2.75%;"|2 !style="width:2.75%;"|3 !style="width:2.75%;"|4 !style="width:2.75%;"|5 !style="width:2.75%;"|6 !style="width:2.75%;"|7 !style="width:2.75%;"|8 !style="width:2.75%;"|9 !style="width:2.75%;"|10 !style="width:2.75%;"|11 !style="width:2.75%;"|12 !style="width:2.75%;"|13 !style="width:2.75%;"|14 !style="width:2.75%;"|15 !style="width:2.75%;"|16 !style="width:2.75%;"|17 !style="width:2.75%;"|18 !style="width:2.75%;"|19 !style="width:2.75%;"|20 !style="width:2.75%;"|21 !style="width:2.75%;"|22 !style="width:2.75%;"|23 !style="width:2.75%;"|24 !style="width:2.75%;"|25 !style="width:2.75%;"|26 !style="width:2.75%;"|27 !style="width:2.75%;"|28 !style="width:2.75%;"|29 !style="width:2.75%;"|30 !style="width:2.75%;"|31 |
| 0
!0 |C | |K |S |colspan="9"|Reserved 0 |colspan="3"|Version |colspan="16"|Protocol Type |
| 4
!32 |colspan="16"|Checksum (optional) |colspan="16"|Reserved 1 (optional) |
| 8
!64 |colspan="32"|Key (optional) |
| 12
!96 |colspan="32"|Sequence Number (optional) |
; 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{{Ref RFC|2784}} is represented in the diagram below.
:
| class="wikitable" style="text-align: center"
|+Standard GRE header format |
| style="border-bottom:none; border-right:none;"|Offsets
!style="border-left:none;"|Octet !colspan="8"|0 !colspan="8"|1 !colspan="8"|2 !colspan="8"|3 |
|---|
| style="border-top: none"|Octet
!Bit !style="width:2.75%;"|0 !style="width:2.75%;"|1 !style="width:2.75%;"|2 !style="width:2.75%;"|3 !style="width:2.75%;"|4 !style="width:2.75%;"|5 !style="width:2.75%;"|6 !style="width:2.75%;"|7 !style="width:2.75%;"|8 !style="width:2.75%;"|9 !style="width:2.75%;"|10 !style="width:2.75%;"|11 !style="width:2.75%;"|12 !style="width:2.75%;"|13 !style="width:2.75%;"|14 !style="width:2.75%;"|15 !style="width:2.75%;"|16 !style="width:2.75%;"|17 !style="width:2.75%;"|18 !style="width:2.75%;"|19 !style="width:2.75%;"|20 !style="width:2.75%;"|21 !style="width:2.75%;"|22 !style="width:2.75%;"|23 !style="width:2.75%;"|24 !style="width:2.75%;"|25 !style="width:2.75%;"|26 !style="width:2.75%;"|27 !style="width:2.75%;"|28 !style="width:2.75%;"|29 !style="width:2.75%;"|30 !style="width:2.75%;"|31 |
| 0
!0 |C |colspan="12"|Reserved 0 |colspan="3"|Version |colspan="16"|Protocol Type |
| 4
!32 |colspan="16"|Checksum (optional) |colspan="16"|Reserved 1 (optional) |
; 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.{{Ref RFC|7676}})
; 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:{{Ref RFC|1701}}
:
| class="wikitable" style="text-align: center"
|+Original GRE header format |
| style="border-bottom:none; border-right:none;"|Offsets
!style="border-left:none;"|Octet !colspan="8"|0 !colspan="8"|1 !colspan="8"|2 !colspan="8"|3 |
|---|
| style="border-top: none"|Octet
!Bit !style="width:2.75%;"|0 !style="width:2.75%;"|1 !style="width:2.75%;"|2 !style="width:2.75%;"|3 !style="width:2.75%;"|4 !style="width:2.75%;"|5 !style="width:2.75%;"|6 !style="width:2.75%;"|7 !style="width:2.75%;"|8 !style="width:2.75%;"|9 !style="width:2.75%;"|10 !style="width:2.75%;"|11 !style="width:2.75%;"|12 !style="width:2.75%;"|13 !style="width:2.75%;"|14 !style="width:2.75%;"|15 !style="width:2.75%;"|16 !style="width:2.75%;"|17 !style="width:2.75%;"|18 !style="width:2.75%;"|19 !style="width:2.75%;"|20 !style="width:2.75%;"|21 !style="width:2.75%;"|22 !style="width:2.75%;"|23 !style="width:2.75%;"|24 !style="width:2.75%;"|25 !style="width:2.75%;"|26 !style="width:2.75%;"|27 !style="width:2.75%;"|28 !style="width:2.75%;"|29 !style="width:2.75%;"|30 !style="width:2.75%;"|31 |
| 0
!0 |C |R |K |S |s |colspan="3"|Recur |colspan="5"|Flags |colspan="3"|Version |colspan="16"|Protocol Type |
| 4
!32 |colspan="16"|Checksum (optional) |colspan="16"|Offset (optional) |
| 8
!64 |colspan="32"|Key (optional) |
| 12
!96 |colspan="32"|Sequence Number (optional) |
| 16
!128 |colspan="32"|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) {{Ref RFC|2637}} uses a variant GRE packet header structure, represented below. PPTP creates a GRE tunnel through which the PPTP GRE packets are sent.
:
| class="wikitable" style="text-align: center"
|+PPTP GRE header format |
| style="border-bottom:none; border-right:none;"|Offsets
!style="border-left:none;"|Octet !colspan="8"|0 !colspan="8"|1 !colspan="8"|2 !colspan="8"|3 |
|---|
| style="border-top: none"|Octet
!Bit !style="width:2.75%;"|0 !style="width:2.75%;"|1 !style="width:2.75%;"|2 !style="width:2.75%;"|3 !style="width:2.75%;"|4 !style="width:2.75%;"|5 !style="width:2.75%;"|6 !style="width:2.75%;"|7 !style="width:2.75%;"|8 !style="width:2.75%;"|9 !style="width:2.75%;"|10 !style="width:2.75%;"|11 !style="width:2.75%;"|12 !style="width:2.75%;"|13 !style="width:2.75%;"|14 !style="width:2.75%;"|15 !style="width:2.75%;"|16 !style="width:2.75%;"|17 !style="width:2.75%;"|18 !style="width:2.75%;"|19 !style="width:2.75%;"|20 !style="width:2.75%;"|21 !style="width:2.75%;"|22 !style="width:2.75%;"|23 !style="width:2.75%;"|24 !style="width:2.75%;"|25 !style="width:2.75%;"|26 !style="width:2.75%;"|27 !style="width:2.75%;"|28 !style="width:2.75%;"|29 !style="width:2.75%;"|30 !style="width:2.75%;"|31 |
| 0
!0 |C |R |K |S |s |colspan="3"|Recur |colspan="1"|A |colspan="4"|Flags |colspan="3"|Version |colspan="16"|Protocol Type |
| 4
!32 |colspan="16"|Key Payload Length |colspan="16"|Key Call ID |
| 8
!64 |colspan="32"|Sequence Number (optional) |
| 12
!96 |colspan="32"|Acknowledgement Number (optional) |
; 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.
Standards
- {{IETF RFC|1701|link=no}}: Generic Routing Encapsulation (GRE) (informational)
- {{IETF RFC|1702|link=no}}: Generic Routing Encapsulation over IPv4 networks (informational)
- {{IETF RFC|2637|link=no}}: Point to Point Tunneling Protocol (informational)
- {{IETF RFC|2784|link=no}}: Generic Routing Encapsulation (GRE) (proposed standard, updated by RFC 2890)
- {{IETF RFC|2890|link=no}}: Key and Sequence Number Extensions to GRE (proposed standard)
- {{IETF RFC|8086|link=no}}: GRE-in-UDP Encapsulation (proposed standard)
See also
- 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
References
{{reflist}}
External links
{{Commons}}
- [http://www.cisco.com/en/US/tech/tk827/tk369/tk287/tsd_technology_support_sub-protocol_home.html Generic Routing Encapsulation], Subprotocol homepage at Cisco
- [http://docwiki.cisco.com/wiki/Internetworking_Terms:_Generic_Routing_Encapsulation_%28GRE%29 Generic Routing Encapsulation] {{Webarchive|url=https://web.archive.org/web/20181229224642/http://docwiki.cisco.com/wiki/Internetworking_Terms:_Generic_Routing_Encapsulation_%28GRE%29 |date=2018-12-29 }}, Entry in Cisco DocWiki (formerly known as the "Internetworking Technology Handbook")