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Network Working Group                                    D. FarinacciRequest for Comments: 2784                                      T. LiCategory: Standards Track                            Procket Networks                                                             S. Hanks                                                 Enron Communications                                                             D. Meyer                                                        Cisco Systems                                                            P. Traina                                                     Juniper Networks                                                           March 2000Generic 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 [RFC1234,RFC1226] currently exist   for the encapsulation of one protocol over another protocol. Other   types of encapsulations [RFC1241,RFC1479] 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 [RFC1326].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   inRFC 2119 [RFC2119].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 20002.2. 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 implementsRFC 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 [RFC1700] as "ETHER   TYPES" and in [ETYPES]. An implementation receiving a packet   containing a Protocol Type which is not listed in [RFC1700] 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 20003.1. 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 [RFC1700] is used when GRE packets are   enapsulated in IPv4. See [RFC1122] for requirements relating to the   delivery of packets over IPv4 networks.5. Interoperation withRFC 1701 Compliant Implementations   InRFC 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 inRFC 1701.   There are, however, existing implementations ofRFC 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. AnRFC 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 theRFC 1701 Key, Sequence Number or Routing fields to be   present.5.2.RFC 1701 Compliant Transmitter   AnRFC 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 theRFC 1701 Key, Sequence Number or   Routing fields in the GRE header. As stated inSection 5.3, a packet   with non-zero bits in any of bits 1-5 MUST be discarded unless the   receiver implementsRFC 1701.Farinacci, et al.           Standards Track                     [Page 4]

RFC 2784             Generic Routing Encapsulation            March 20006. 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 inRFC 2434 [RFC2434].7.2.  Protocol Types   GRE uses an ETHER Type for the Protocol Type. New ETHER TYPES are   assigned by Xerox Systems Institute [RFC1700].8. Acknowledgments   This document is derived from the original ideas of the authors ofRFC 1701 andRFC 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 20009. 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) [RFC1191]     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 Encapsulations10. REFERENCES   [ETYPES]ftp://ftp.isi.edu/in-notes/iana/assignments/ethernet-numbers   [RFC1122] Braden, R., "Requirements for Internet hosts -             communication layers", STD 3,RFC 1122, October 1989.   [RFC1191] 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   [RFC1226] Kantor, B., "Internet Protocol Encapsulation of AX.25             Frames",RFC 1226, May 1991.   [RFC1234] Provan, D., "Tunneling IPX Traffic through IP Networks",RFC 1234, June 1991.   [RFC1241] Woodburn, R. and D. Mills, "Scheme for an Internet             Encapsulation Protocol: Version 1",RFC 1241, July 1991.   [RFC1326] Tsuchiya, P., "Mutual Encapsulation Considered Dangerous",RFC 1326, May 1992.   [RFC1479] Steenstrup, M., "Inter-Domain Policy Routing Protocol             Specification: Version 1",RFC 1479, July 1993.   [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,RFC1700, October 1994.   [RFC1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic             Routing Encapsulation",RFC 1701, October 1994.   [RFC1702] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic             Routing Encapsulation over IPv4 networks",RFC 1702,             October 1994.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March, 1997.   [RFC2408] Maughan, D., Schertler, M., Schneider, M. and J.  Turner,             "Internet Security Association and Key Management Protocol             (ISAKMP)",RFC 2408, November 1998.   [RFC2434] 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)",RFC 2637, July, 1999.Farinacci, et al.           Standards Track                     [Page 7]

RFC 2784             Generic Routing Encapsulation            March 200011.  Authors' Addresses   Dino Farinacci   Procket Networks   3850 No. First St., Ste. C   San Jose, CA 95134   EMail: dino@procket.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: tony1@home.net   Stan Hanks   Enron Communications   EMail: stan_hanks@enron.net   David Meyer   Cisco Systems, Inc.   170 Tasman Drive   San Jose, CA, 95134   EMail: dmm@cisco.com   Paul Traina   Juniper Networks   EMail: pst@juniper.netFarinacci, et al.           Standards Track                     [Page 8]

RFC 2784             Generic Routing Encapsulation            March 200012.  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]