View source for 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 = [[Tunneling protocol|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 [[Encapsulation (networking)|encapsulate]] a wide variety of [[network layer]] [[Protocol (computing)|protocols]] inside virtual [[Point-to-point (telecommunications)|point-to-point links]] or point-to-multipoint links over an [[Internet Protocol]] network.<ref>{{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|Cisco Technology Inc.]]|gdate=2010-09-21|fdate=2002-07-01|url=https://patents.google.com/patent/US7801021B1/en}}</ref>

==Example uses==
* In conjunction with [[Point-to-point tunneling protocol|PPTP]] to create [[VPN]]s.
* In conjunction with [[IPsec]] [[VPN]]s to allow passing of routing information between connected networks.
* In [[mobility management]] protocols.
* In [[A12 Authentication|A8/A10]] interfaces to encapsulate IP data to/from Packet Control Function (PCF).
* [[Linux]] and [[Berkeley Software Distribution|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 [[Communication endpoint|endpoint]].

===Example protocol stack===
{|class="wikitable"
![[OSI model]] layer
![[Protocol (computing)|Protocol example]]
|-
|7. [[Application layer|Application]]
|[[HTTP]]
|-
|4. [[Transport layer|Transport]]
|[[Transmission Control Protocol|TCP]]
|-
|3. [[Network layer|Network]] <small>(GRE-encapsulated)</small>
|[[IPv4]]
|-
|[[Encapsulation (networking)|Encapsulation]]
|'''GRE'''
|-
|3. [[Network layer|Network]]
|[[IPv6]]
|-
|2. [[Data link layer|Data link]]
|[[Ethernet]]
|-
|1. [[Physical layer|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.

==Delivery protocols==
GRE packets that are encapsulated within [[Internet Protocol|IP]] directly, use IP protocol [[List of IP protocol numbers|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 [[Header (computing)|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 (computing)|Octet]]
!colspan="8"|0
!colspan="8"|1
!colspan="8"|2
!colspan="8"|3
|-
!style="border-top: none"|[[Octet (computing)|Octet]]
![[Bit]]
!style="width:2.75%;"|0
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|-
!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 [[EtherType|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 [[Header (computing)|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 (computing)|Octet]]
!colspan="8"|0
!colspan="8"|1
!colspan="8"|2
!colspan="8"|3
|-
!style="border-top: none"|[[Octet (computing)|Octet]]
![[Bit]]
!style="width:2.75%;"|0
!style="width:2.75%;"|1
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|-
!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 [[EtherType|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 (computing)|Octet]]
!colspan="8"|0
!colspan="8"|1
!colspan="8"|2
!colspan="8"|3
|-
!style="border-top: none"|[[Octet (computing)|Octet]]
![[Bit]]
!style="width:2.75%;"|0
!style="width:2.75%;"|1
!style="width:2.75%;"|2
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|-
!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 (computer science)|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 [[EtherType|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 (computing)|Octet]]
!colspan="8"|0
!colspan="8"|1
!colspan="8"|2
!colspan="8"|3
|-
!style="border-top: none"|[[Octet (computing)|Octet]]
![[Bit]]
!style="width:2.75%;"|0
!style="width:2.75%;"|1
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|-
!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 (computer science)|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")

[[Category:Tunneling protocols]]
[[Category:Cisco Systems]]