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 protocolsedit
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.
Packet headeredit
Extended GRE packet header (RFC 2890)edit
The extended version of the GRE packet header[6] is represented below:
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 headeredit
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.
^ abS. Hanks; T. Li; D. Farinacci; P. Traina (October 1994). Generic Routing Encapsulation (GRE). Network Working Group. doi:10.17487/RFC1701. RFC 1701. Informational.
^US 7801021B1, Nikolaos Triantafillis; Robert J. Ordemann & Simon D. Barber, "Generic routing encapsulation tunnel keepalives", issued 2010-09-21, assigned to Cisco Technology Inc.
^S. Hanks; T. Li; D. Farinacci; P. Traina (October 1994). Generic Routing Encapsulation over IPv4 networks. Network Working Group. doi:10.17487/RFC1702. RFC 1702. Informational.
^ abC. Pignataro; R. Bonica; S. Krishnan (October 2015). IPv6 Support for Generic Routing Encapsulation (GRE). Internet Engineering Task Force (IETF). doi:10.17487/RFC7676. ISSN 2070-1721. RFC 7676. Proposed Standard.
^E. Crabbe; E. Crabbet; T. Herbert (March 2017). L. Yong (ed.). GRE-in-UDP Encapsulation. Internet Engineering Task Force (IETF). doi:10.17487/RFC8086. ISSN 2070-1721. RFC 8086. Proposed Standard.
^G. Dommety (September 2000). Key and Sequence Number Extensions to GRE. Network Working Group. doi:10.17487/RFC2890. RFC 2890. Proposed Standard.
^D. Farinacci; T. Li; S. Hanks; D. Meyer; P. Traina (March 2000). Generic Routing Encapsulation (GRE). Network Working Group. doi:10.17487/RFC2784. RFC 2784. Proposed Standard. Updated by RFC 2890.
^K. Hamzeh; G. Pall; W. Verthein; J. Taarud; W. Little; G. Zorn (July 1999). Point-to-Point Tunneling Protocol (PPTP). Network Working Group. doi:10.17487/RFC2637. RFC 2637. Informational.
External linksedit
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")
October 26, 2023
generic, routing, encapsulation, tunneling, protocol, developed, cisco, systems, that, encapsulate, wide, variety, network, layer, protocols, inside, virtual, point, point, links, point, multipoint, links, over, internet, protocol, network, communication, prot. 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 Generic Routing EncapsulationCommunication protocolPurposeNetwork tunnelingDeveloper s Cisco SystemsIntroduction1994 1 RFC s RFC1701 RFC1702 RFC2784 Contents 1 Example uses 1 1 Example protocol stack 2 Delivery protocols 3 Packet header 3 1 Extended GRE packet header RFC 2890 3 2 Standard GRE packet header RFC 2784 3 3 Original GRE packet header RFC 1701 3 4 PPTP GRE packet header 4 Standards 5 See also 6 References 7 External linksExample uses editIn 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 edit OSI model layer Protocol7 Session X 2256 Transport UDP5 Network GRE encapsulated IPv44 Encapsulation GRE3 Network IPv62 Data link Ethernet1 Physical Ethernet physical layerBased 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 editGRE 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 Packet header editExtended GRE packet header RFC 2890 edit The extended version of the GRE packet header 6 is represented below Extended GRE header format Offsets Octet 0 1 2 3Octet 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 30 310 0 C K S Reserved 0 Version Protocol Type4 32 Checksum optional Reserved 1 optional 8 64 Key optional 12 96 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 edit A standard GRE packet header structure 7 is represented in the diagram below Standard GRE header format Offsets Octet 0 1 2 3Octet 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 30 310 0 C Reserved 0 Version Protocol Type4 32 Checksum optional 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 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 edit The newer structure superseded the original structure 1 Original GRE header format Offsets Octet 0 1 2 3Octet 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 30 310 0 C R K S s Recur Flags Version Protocol Type4 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 edit 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 PPTP GRE header format Offsets Octet 0 1 2 3Octet 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 30 310 0 C R K S s Recur A Flags Version Protocol Type4 32 Key Payload Length Key Call ID8 64 Sequence Number optional 12 96 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 editRFC 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 editNetwork Virtualization using Generic Routing Encapsulation carries L2 packets over GREReferences edit a b S Hanks T Li D Farinacci P Traina October 1994 Generic Routing Encapsulation GRE Network Working Group doi 10 17487 RFC1701 RFC 1701 Informational US 7801021B1 Nikolaos Triantafillis Robert J Ordemann amp Simon D Barber Generic routing encapsulation tunnel keepalives issued 2010 09 21 assigned to Cisco Technology Inc S Hanks T Li D Farinacci P Traina October 1994 Generic Routing Encapsulation over IPv4 networks Network Working Group doi 10 17487 RFC1702 RFC 1702 Informational a b C Pignataro R Bonica S Krishnan October 2015 IPv6 Support for Generic Routing Encapsulation GRE Internet Engineering Task Force IETF doi 10 17487 RFC7676 ISSN 2070 1721 RFC 7676 Proposed Standard E Crabbe E Crabbet T Herbert March 2017 L Yong ed GRE in UDP Encapsulation Internet Engineering Task Force IETF doi 10 17487 RFC8086 ISSN 2070 1721 RFC 8086 Proposed Standard G Dommety September 2000 Key and Sequence Number Extensions to GRE Network Working Group doi 10 17487 RFC2890 RFC 2890 Proposed Standard D Farinacci T Li S Hanks D Meyer P Traina March 2000 Generic Routing Encapsulation GRE Network Working Group doi 10 17487 RFC2784 RFC 2784 Proposed Standard Updated by RFC 2890 K Hamzeh G Pall W Verthein J Taarud W Little G Zorn July 1999 Point to Point Tunneling Protocol PPTP Network Working Group doi 10 17487 RFC2637 RFC 2637 Informational External links edit nbsp 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 Retrieved from https en wikipedia org w index php title Generic Routing Encapsulation amp oldid 1181452926, wikipedia, wiki, book, books, library,
