Generic Routing Encapsulation (GRE)
Author(s): T. Li, P. Traina, D. Meyer, S. Hanks, D. Farinacci
This document specifies a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol. [STANDARDS...
Network Working Group D. Farinacci Request for Comments: 2784 T. Li Category: Standards Track Procket Networks S. Hanks Enron Communications D. Meyer Cisco Systems P. Traina Juniper Networks March 2000 Generic Routing Encapsulation (GRE) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract This document specifies a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol. 1. Introduction A number of different proposals [RFC 1234, RFC 1226] currently exist for the encapsulation of one protocol over another protocol. Other types of encapsulations [RFC 1241, RFC 1479] have been proposed for transporting IP over IP for policy purposes. This memo describes a protocol which is very similar to, but is more general than, the above proposals. In attempting to be more general, many protocol specific nuances have been ignored. The result is that this proposal may be less suitable for a situation where a specific "X over Y" encapsulation has been described. It is the attempt of this protocol to provide a simple, general purpose mechanism which reduces the problem of encapsulation from its current O(n^2) size to a more manageable size. This memo purposely does not address the issue of when a packet should be encapsulated. This memo acknowledges, but does not address problems such as mutual encapsulation [RFC 1326]. Farinacci, et al. Standards Track [Page 1] RFC 2784 Generic Routing Encapsulation March 2000 In the most general case, a system has a packet that needs to be encapsulated and delivered to some destination. We will call this the payload packet. The payload is first encapsulated in a GRE packet. The resulting GRE packet can then be encapsulated in some other protocol and then forwarded. We will call this outer protocol the delivery protocol. The algorithms for processing this packet are discussed later. Finally this specification describes the intersection of GRE currently deployed by multiple vendors. The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined in RFC 2119 [RFC 2119]. 2. Structure of a GRE Encapsulated Packet A GRE encapsulated packet has the form: --------------------------------- | | | Delivery Header | | | --------------------------------- | | | GRE Header | | | --------------------------------- | | | Payload packet | | | --------------------------------- This specification is generally concerned with the structure of the GRE header, although special consideration is given to some of the issues surrounding IPv4 payloads. 2.1. GRE Header The GRE packet header has the form: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C| Reserved0 | Ver | Protocol Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum (optional) | Reserved1 (Optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Farinacci, et al. Standards Track [Page 2] RFC 2784 Generic Routing Encapsulation March 2000 2.2. Checksum Present (bit 0) If the Checksum Present bit is set to one, then the Checksum and the Reserved1 fields are present and the Checksum field contains valid information. Note that a compliant implementation MUST accept and process this field. 2.3. Reserved0 (bits 1-12) A receiver MUST discard a packet where any of bits 1-5 are non-zero, unless that receiver implements RFC 1701. Bits 6-12 are reserved for future use. These bits MUST be sent as zero and MUST be ignored on receipt. 2.3.1. Version Number (bits 13-15) The Version Number field MUST contain the value zero. 2.4. Protocol Type (2 octets) The Protocol Type field contains the protocol type of the payload packet. These Protocol Types are defined in [RFC 1700] as "ETHER TYPES" and in [ETYPES]. An implementation receiving a packet containing a Protocol Type which is not listed in [RFC 1700] or [ETYPES] SHOULD discard the packet. 2.5. Checksum (2 octets) The Checksum field contains the IP (one's complement) checksum sum of the all the 16 bit words in the GRE header and the payload packet. For purposes of computing the checksum, the value of the checksum field is zero. This field is present only if the Checksum Present bit is set to one. 2.6. Reserved1 (2 octets) The Reserved1 field is reserved for future use, and if present, MUST be transmitted as zero. The Reserved1 field is present only when the Checksum field is present (that is, Checksum Present bit is set to one). 3. IPv4 as a Payload When IPv4 is being carried as the GRE payload, the Protocol Type field MUST be set to 0x800. Farinacci, et al. Standards Track [Page 3] RFC 2784 Generic Routing Encapsulation March 2000 3.1. Forwarding Decapsulated IPv4 Payload Packets When a tunnel endpoint decapsulates a GRE packet which has an IPv4 packet as the payload, the destination address in the IPv4 payload packet header MUST be used to forward the packet and the TTL of the payload packet MUST be decremented. Care should be taken when forwarding such a packet, since if the destination address of the payload packet is the encapsulator of the packet (i.e., the other end of the tunnel), looping can occur. In this case, the packet MUST be discarded. 4. IPv4 as a Delivery Protocol The IPv4 protocol 47 [RFC 1700] is used when GRE packets are enapsulated in IPv4. See [RFC 1122] for requirements relating to the delivery of packets over IPv4 networks. 5. Interoperation with RFC 1701 Compliant Implementations In RFC 1701, the field described here as Reserved0 contained a number of flag bits which this specification deprecates. In particular, the Routing Present, Key Present, Sequence Number Present, and Strict Source Route bits have been deprecated, along with the Recursion Control field. As a result, the GRE header will never contain the Key, Sequence Number or Routing fields specified in RFC 1701. There are, however, existing implementations of RFC 1701. The following sections describe correct interoperation with such implementations. 5.1. RFC 1701 Compliant Receiver An implementation complying to this specification will transmit the Reserved0 field set to zero. An RFC 1701 compliant receiver will interpret this as having the Routing Present, Key Present, Sequence Number Present, and Strict Source Route bits set to zero, and will not expect the RFC 1701 Key, Sequence Number or Routing fields to be present. 5.2. RFC 1701 Compliant Transmitter An RFC 1701 transmitter may set any of the Routing Present, Key Present, Sequence Number Present, and Strict Source Route bits set to one, and thus may transmit the RFC 1701 Key, Sequence Number or Routing fields in the GRE header. As stated in Section 5.3, a packet with non-zero bits in any of bits 1-5 MUST be discarded unless the receiver implements RFC 1701. Farinacci, et al. Standards Track [Page 4] RFC 2784 Generic Routing Encapsulation March 2000 6. Security Considerations Security in a network using GRE should be relatively similar to security in a normal IPv4 network, as routing using GRE follows the same routing that IPv4 uses natively. Route filtering will remain unchanged. However packet filtering requires either that a firewall look inside the GRE packet or that the filtering is done on the GRE tunnel endpoints. In those environments in which this is considered to be a security issue it may be desirable to terminate the tunnel at the firewall. 7. IANA Considerations This section considers the assignment of additional GRE Version Numbers and Protocol Types. 7.1. GRE Version Numbers This document specifies GRE version number 0. GRE version number 1 is used by PPTP [RFC2637]. Additional GRE version numbers are assigned by IETF Consensus as defined in RFC 2434 [RFC 2434]. 7.2. Protocol Types GRE uses an ETHER Type for the Protocol Type. New ETHER TYPES are assigned by Xerox Systems Institute [RFC 1700]. 8. Acknowledgments This document is derived from the original ideas of the authors of RFC 1701 and RFC 1702. Hitoshi Asaeda, Scott Bradner, Randy Bush, Brian Carpenter, Bill Fenner, Andy Malis, Thomas Narten, Dave Thaler, Tim Gleeson and others provided many constructive and insightful comments. Farinacci, et al. Standards Track [Page 5] RFC 2784 Generic Routing Encapsulation March 2000 9. Appendix -- Known Issues This document specifies the behavior of currently deployed GRE implementations. As such, it does not attempt to address the following known issues: o Interaction Path MTU Discovery (PMTU) [RFC 1191] Existing implementations of GRE, when using IPv4 as the Delivery Header, do not implement Path MTU discovery and do not set the Don't Fragment bit in the Delivery Header. This can cause large packets to become fragmented within the tunnel and reassembled at the tunnel exit (independent of whether the payload packet is using PMTU). If a tunnel entry point were to use Path MTU discovery, however, that tunnel entry point would also need to relay ICMP unreachable error messages (in particular the "fragmentation needed and DF set" code) back to the originator of the packet, which is not a requirement in this specification. Failure to properly relay Path MTU information to an originator can result in the following behavior: the originator sets the don't fragment bit, the packet gets dropped within the tunnel, but since the originator doesn't receive proper feedback, it retransmits with the same PMTU, causing subsequently transmitted packets to be dropped. o IPv6 as Delivery and/or Payload Protocol This specification describes the intersection of GRE currently deployed by multiple vendors. IPv6 as delivery and/or payload protocol is not included in the currently deployed versions of GRE. o Interaction with ICMP o Interaction with the Differentiated Services Architecture o Multiple and Looping Encapsulations 10. REFERENCES [ETYPES] ftp://ftp.isi.edu/in-notes/iana/assignments/ethernet- numbers [RFC 1122] Braden, R., "Requirements for Internet hosts - communication layers", STD 3, RFC 1122, October 1989. [RFC 1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191, November 1990. Farinacci, et al. Standards Track [Page 6] RFC 2784 Generic Routing Encapsulation March 2000 [RFC 1226] Kantor, B., "Internet Protocol Encapsulation of AX.25 Frames", RFC 1226, May 1991. [RFC 1234] Provan, D., "Tunneling IPX Traffic through IP Networks", RFC 1234, June 1991. [RFC 1241] Woodburn, R. and D. Mills, "Scheme for an Internet Encapsulation Protocol: Version 1", RFC 1241, July 1991. [RFC 1326] Tsuchiya, P., "Mutual Encapsulation Considered Dangerous", RFC 1326, May 1992. [RFC 1479] Steenstrup, M., "Inter-Domain Policy Routing Protocol Specification: Version 1", RFC 1479, July 1993. [RFC 1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. [RFC 1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation", RFC 1701, October 1994. [RFC 1702] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation over IPv4 networks", RFC 1702, October 1994. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March, 1997. [RFC 2408] Maughan, D., Schertler, M., Schneider, M. and J. Turner, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October, 1998. [RFC2637] Hamzeh, K., et al., "Point-to-Point Tunneling Protocol (PPTP)", RFC2637, July, 1999. Farinacci, et al. Standards Track [Page 7] RFC 2784 Generic Routing Encapsulation March 2000 11. Authors' Addresses Dino Farinacci Procket Networks 3850 No. First St., Ste. C San Jose, CA 95134 EMail: email@example.com Tony Li Procket Networks 3850 No. First St., Ste. C San Jose, CA 95134 Phone: +1 408 954 7903 Fax: +1 408 987 6166 EMail: firstname.lastname@example.org Stan Hanks Enron Communications EMail: email@example.com David Meyer Cisco Systems, Inc. 170 Tasman Drive San Jose, CA, 95134 EMail: firstname.lastname@example.org Paul Traina Juniper Networks EMail: email@example.com Farinacci, et al. Standards Track [Page 8] RFC 2784 Generic Routing Encapsulation March 2000 12. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Farinacci, et al. Standards Track [Page 9]