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rfc2473 Packet Tunneling in IPv6

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rfc2473 Packet Tunneling in IPv6 ...

Network Working Group A. Conta

    Request for Comments: 2473 Lucent Technologies Inc.

    Category: Standards Track S. Deering

    Cisco Systems

    December 1998

    Generic Packet Tunneling in IPv6

    Specification

    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 (1998). All Rights Reserved. Abstract

    This document defines the model and generic mechanisms for IPv6 encapsulation of Internet packets, such as IPv6 and IPv4. The model and mechanisms can be applied to other protocol packets as well, such as AppleTalk, IPX, CLNP, or others.

    Table of Contents

    1. Introduction..................................................2 2. Terminology...................................................2

    3. IPv6 Tunneling................................................4 3.1 IPv6 Encapsulation.......................................6 3.2 IPv6 Packet Processing in Tunnels........................7 3.3 IPv6 Decapsulation.......................................7 3.4 IPv6 Tunnel Protocol Engine..............................8 4. Nested Encapsulation.........................................11 4.1 Limiting Nested Encapsulation..........................12 4.1.1 Tunnel Encapsulation Limit Option................13 4.1.2 Loopback Encapsulation...........................15 4.1.3 Routing Loop Nested Encapsulation................15 5. Tunnel IPv6 Header...........................................16 5.1 Tunnel IPv6 Extension Headers...........................17 6. IPv6 Tunnel State Variables..................................19 6.1 IPv6 Tunnel Entry-Point Node............................19 6.2 IPv6 Tunnel Exit-Point Node.............................19 Conta & Deering Standards Track [Page 1]

    RFC 2473 Generic Packet Tunneling in IPv6 December 1998 6.3 IPv6 Tunnel Hop Limit...................................19 6.4 IPv6 Tunnel Packet Traffic Class........................20 6.5 IPv6 Tunnel Flow Label..................................20 6.6 IPv6 Tunnel Encapsulation Limit.........................20 6.7 IPv6 Tunnel MTU.........................................20 7. IPv6 Tunnel Packet Size Issues...............................21 7.1 IPv6 Tunnel Packet Fragmentation........................21 7.2 IPv4 Tunnel Packet Fragmentation........................22 8. IPv6 Tunnel Error Reporting and Processing...................22 8.1 Tunnel ICMP Messages....................................27 8.2 ICMP Messages for IPv6 Original Packets.................28 8.3 ICMP Messages for IPv4 Original Packets.................29 8.4 ICMP Messages for Nested Tunnel Packets.................30 9. Security Considerations......................................30 10. Acknowledgments.............................................31 11. References..................................................31 Authors' Addresses..............................................32 Appendix A. Risk Factors in Recursive Encapsulation.............33 Full Copyright Statement........................................36 1. Introduction

    This document specifies a method and generic mechanisms by which a

    packet is encapsulated and carried as payload within an IPv6 packet. The resulting packet is called an IPv6 tunnel packet. The forwarding path between the source and destination of the tunnel packet is called an IPv6 tunnel. The technique is called IPv6 tunneling. A typical scenario for IPv6 tunneling is the case in which an intermediate node exerts explicit routing control by specifying particular forwarding paths for selected packets. This control is achieved by prepending IPv6 headers to each of the selected original packets. These prepended headers identify the forwarding paths. In addition to the description of generic IPv6 tunneling mechanisms, which is the focus of this document, specific mechanisms for tunneling IPv6 and IPv4 packets are also described herein. The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED,

    RECOMMENDED,

    SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined in RFC 2119.

    2. Terminology

    original packet

    a packet that undergoes encapsulation.

    Conta & Deering Standards Track [Page 2]

    RFC 2473 Generic Packet Tunneling in IPv6 December 1998 original header

    the header of an original packet.

    tunnel

    a forwarding path between two nodes on which the payloads of packets are original packets.

    tunnel end-node

    a node where a tunnel begins or ends.

    tunnel header

    the header prepended to the original packet during encapsulation. It specifies the tunnel end-points as source and destination.

    tunnel packet

    a packet that encapsulates an original packet.

    tunnel entry-point

    the tunnel end-node where an original packet is encapsulated. tunnel exit-point

    the tunnel end-node where a tunnel packet is decapsulated. IPv6 tunnel

    a tunnel configured as a virtual link between two IPv6 nodes, on which the encapsulating protocol is IPv6.

    tunnel MTU

    the maximum size of a tunnel packet payload without requiring fragmentation, that is, the Path MTU between the tunnel entry- point and the tunnel exit-point nodes minus the size of the tunnel header.

    tunnel hop limit

    the maximum number of hops that a tunnel packet can travel from the tunnel entry-point to the tunnel exit-point.

    Conta & Deering Standards Track [Page 3]

    RFC 2473 Generic Packet Tunneling in IPv6 December 1998 inner tunnel

    a tunnel that is a hop (virtual link) of another tunnel. outer tunnel

    a tunnel containing one or more inner tunnels.

    nested tunnel packet

    a tunnel packet that has as payload a tunnel packet. nested tunnel header

    the tunnel header of a nested tunnel packet.

    nested encapsulation

    encapsulation of an encapsulated packet.

    recursive encapsulation

    encapsulation of a packet that reenters a tunnel before exiting it.

    tunnel encapsulation limit

    the maximum number of nested encapsulations of a packet. 3. IPv6 Tunneling

    IPv6 tunneling is a technique for establishing a "virtual link" between two IPv6 nodes for transmitting data packets as payloads of IPv6 packets (see Fig.1). From the point of view of the two nodes, this "virtual link", called an IPv6 tunnel, appears as a point to point link on which IPv6 acts like a link-layer protocol. The two

    IPv6 nodes play specific roles. One node encapsulates original packets received from other nodes or from itself and forwards the resulting tunnel packets through the tunnel. The other node decapsulates the received tunnel packets and forwards the resulting original packets towards their destinations, possibly itself. The encapsulator node is called the tunnel entry-point node, and it is the source of the tunnel packets. The decapsulator node is called the tunnel exit-point, and it is the destination of the tunnel packets. Conta & Deering Standards Track [Page 4]

    RFC 2473 Generic Packet Tunneling in IPv6 December 1998 Note:

    This document refers in particular to tunnels between two nodes identified by unicast addresses - such tunnels look like "virtual point to point links". The mechanisms described herein apply also to tunnels in which the exit-point nodes are identified by other types of addresses, such as anycast or multicast. These tunnels may look like "virtual point to multipoint links". At the time of writing this document, IPv6 anycast addresses are a subject of ongoing specification and experimental work.

    Tunnel from node B to node C

    Tunnel Tunnel

    Entry-Point Exit-Point

    Node Node

    +-+ +-+ +-+ +-+

    |A|-->--//-->--|B|=====>=====//=====>=====|C|-->--//-->--|D| +-+ +-+ +-+ +-+

    Original Original

    Packet Packet

    Source Destination

    Node Node

    Fig.1 Tunnel

An IPv6 tunnel is a unidirectional mechanism - tunnel packet flow

    takes place in one direction between the IPv6 tunnel entry-point and

    exit-point nodes (see Fig.1).

    Tunnel from Node B to Node C

    Tunnel Tunnel

    Original Entry-Point Exit-Point Original Packet Node Node Packet

    Source Destination

    Node Node

    +-+ +-+ +-+ +-+

    | |-->--//-->--| |=====>=====//=====>======| |-->--//-->--| |

    |A| |B| |C| |D|

    | |--

    +-+ +-+ +-+ +-+

    Original Original

    Packet Packet

    Destination Tunnel Tunnel Source Node Exit-Point Entry-Point Node Node Node

    Tunnel from Node C to Node B

    Fig.2 Bi-directional Tunneling Mechanism Conta & Deering Standards Track [Page 5]

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