Generic Packet Tunneling in IPv6 Specification
draft-ietf-ipngwg-ipv6-tunnel-07

IPng Working Group A. Conta (Lucent Technologies Inc.)
INTERNET-DRAFT S. Deering (Cisco Systems)
 December 1996
 Generic Packet Tunneling in IPv6
 Specification
 draft-ietf-ipngwg-ipv6-tunnel-07.txt
Status of this Memo
 This document is an Internet Draft. Internet Drafts are working
 documents of the Internet Engineering Task Force (IETF), its Areas,
 and its Working Groups. Note that other groups may also distribute
 working documents as Internet Drafts.
 Internet Drafts are draft documents valid for a maximum of six
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 draft" or "work in progress."
 To learn the current status of any Internet-Draft, please check the
 ``1id-abstracts.txt'' listing contained in the Internet- Drafts
 Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net
 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
 Rim).
 Distribution of this memo is unlimited.
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.
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Table of Contents
 Status of this Memo...........................................1
 Table of Contents.............................................2
1. Introduction..................................................3
2. Terminology...................................................3
3. Generic IPv6 Tunneling........................................5
 3.1 IPv6 Encapsulation.......................................7
 3.2 IPv6 Packet Processing in Tunnels........................8
 3.3 IPv6 Decapsulation.......................................8
 3.4 IPv6 Tunnel Protocol Engine..............................9
4. Nested Encapsulation.........................................12
 4.1 Limiting Nested Encapsulation..........................13
 4.1.1 Tunnel Encapsulation Limit.......................14
 4.1.2 Loopback Encapsulation...........................15
 4.1.3 Routing Loop Nested Encapsulation................16
5. Tunnel IPv6 Header...........................................16
 5.1 Tunnel IPv6 Extension Headers...........................18
6. IPv6 Tunnel State Variables..................................19
 6.1 IPv6 Tunnel Entry-Point Node............................19
 6.2 IPv6 Tunnel Exit-Point Node.............................20
 6.3 IPv6 Tunnel Hop Limit...................................20
 6.4 IPv6 Tunnel Packet Priority.............................21
 6.5 IPv6 Tunnel Flow Label..................................21
 6.6 IPv6 Tunnel Encapsulation Limit.........................21
 6.7 IPv6 Tunnel MTU.........................................22
7. IPv6 Tunnel Packet Size Issues...............................22
 7.1 IPv6 Tunnel Packet Fragmentation........................23
 7.2 IPv4 Tunnel Packet Fragmentation........................23
8. IPv6 Tunnel Error Reporting and Processing...................24
 8.1 Tunnel ICMP Messages....................................28
 8.2 ICMP Messages for IPv6 Original Packets.................29
 8.3 ICMP Messages for IPv4 Original Packets.................30
 8.4 ICMP Messages for Nested Tunnel Packets.................31
9. Security Considerations......................................31
10. Acknowledgments.............................................32
11. References..................................................32
Authors' Addresses..............................................33
Appendix A.Risk Factors in Recursive Encapsulation..............34
Fig.1.................................................6
Fig.2.................................................6
Fig.3.................................................7
Fig.4.................................................8
Fig.5.................................................9
Fig.6................................................13
Fig.7................................................25
Fig.8................................................26/27
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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 to each of the selected original packets IPv6
 headers that identify the forwarding path.
 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.
2. Terminology
 original packet
 a packet that undergoes encapsulation.
 original header
 the header of an original packet.
 tunnel
 a forwarding path between two nodes on which packets payloads
 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.
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 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.
 fixed-exit tunnel
 a tunnel for which a specific exit-point was configured.
 free-exit tunnel
 a tunnel for which no specific exit-point was configured; the
 exit point is extracted from the destination of each packet
 encapsulated and sent into the tunnel.
 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 headers.
 tunnel hop limit
 the maximum number of hops that a tunnel packet can travel from
 the tunnel entry-point to the tunnel exit-point.
 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
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 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.
 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.
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 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).
 Bi-directional tunneling is achieved by merging two unidirectional
 mechanisms, that is, configuring two tunnels, each in opposite
 direction to the other - the entry-point node of one tunnel is the
 exit-point node of the other tunnel (see Fig.2).
 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
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3.1 IPv6 Encapsulation
 IPv6 encapsulation consists of prepending to the original packet an
 IPv6 header and, optionally, a set of IPv6 extension headers (see
 Fig.3), which are collectively called tunnel IPv6 headers. The
 encapsulation takes place in an IPv6 tunnel entry-point node, as the
 result of an original packet being forwarded onto the virtual link
 represented by the tunnel. The original packet is processed during
 forwarding according to the forwarding rules of the protocol of that
 packet. For instance if the original packet is an:
 (a) IPv6 packet, the IPv6 original header hop limit is decremented
 by one.
 (b) IPv4 packet, the IPv4 original header time to live field (TTL)
 is decremented by one.
 At encapsulation, the source field of the tunnel IPv6 header is
 filled with an IPv6 address of the tunnel entry-point node, and the
 destination field with an IPv6 address of the tunnel exit-point.
 Subsequently, the tunnel packet resulting from encapsulation is sent
 towards the tunnel exit-point node.
 Tunnel extension headers should appear in the order recommended by
 the specifications that define the extension headers, such as [RFC-
 1883].
 A source of original packets and a tunnel entry-point that
 encapsulates those packets can be the same node.
 +----------------------------------//-----+
 | Original | |
 | | Original Packet Payload |
 | Header | |
 +----------------------------------//-----+
 < Original Packet >
 |
 v
 <Tunnel IPv6 Headers> < Original Packet >
 +---------+ - - - - - +-------------------------//--------------+
 | IPv6 | IPv6 | |
 | | Extension | Original Packet |
 | Header | Headers | |
 +---------+ - - - - - +-------------------------//--------------+
 < Tunnel IPv6 Packet >
 Fig.3 Encapsulating a Packet
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3.2 Packet Processing in Tunnels
 The intermediate nodes in the tunnel process the IPv6 tunnel packets
 according to the IPv6 protocol. For example, a tunnel Hop by Hop
 Options extension header is processed by each receiving node in the
 tunnel; a tunnel Routing extension header identifies the intermediate
 processing nodes, and controls at a finer granularity the forwarding
 path of the tunnel packet through the tunnel; a tunnel Destination
 Options extension header is processed at the tunnel exit-point node.
3.3 IPv6 Decapsulation
 Decapsulation is graphically shown in Fig.4:
 +---------+- - - - - -+----------------------------------//-----+
 | IPv6 | IPv6 | |
 | | Extension | Original Packet |
 | Header | Headers | |
 +---------+- - - - - -+----------------------------------//-----+
 < Tunnel IPv6 Packet >
 |
 v
 +----------------------------------//-----+
 | Original | |
 | | Original Packet Payload |
 | Headers | |
 +----------------------------------//-----+
 < Original Packet >
 Fig.4 Decapsulating a Packet
 Upon receiving an IPv6 packet destined to an IPv6 address of a tunnel
 exit-point node, its IPv6 protocol layer processes the tunnel
 headers. The strict left-to-right processing rules for extension
 headers is applied. When processing is complete, control is handed to
 the next protocol engine, which is identified by the Next Header
 field value in the last header processed. If this is set to a tunnel
 protocol value, the tunnel protocol engine discards the tunnel
 headers and passes the resulting original packet to the Internet or
 lower layer protocol identified by that value for further processing.
 For example, in the case the Next Header field has the IPv6 Tunnel
 Protocol value, the resulting original packet is passed to the IPv6
 protocol layer.
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 The tunnel exit-point node, which decapsulates the tunnel packets,
 and the destination node, which receives the resulting original
 packets can be the same node.
3.4 IPv6 Tunnel Protocol Engine
 Packet flow (paths #1-7) through the IPv6 Tunnel Protocol Engine on a
 node is graphically shown in Fig.5:
 +-----------------------+ +-----------------------------------+
 | Upper-Layer Protocols | | IPv6 Tunnel Upper-Layer |
 | | | |
 | | | ---<-------------------<------- |
 | | | | ---->---|------>--------- | |
 | | | | | | | | | |
 +-----------------------+ +-----------------------+ | | |
 | | | | | | | | | v ^ |
 v ^ v ^ v ^ v ^ Tunnel | | | |
 | | | | | | | | Packets| | | |
 +---------------------------------------------+ | | | |
 | | | | | / / | | | | D E |
 | v ^ IPv6 | --<-3--/-/--<---- | | | | E N |
 | | | Layer ---->-4-/-/--->-- | | | | | C C |
 | v ^ / / | | | | | | A A |
 | | | 2 1 | | | | | | P P |
 | v ^ -----<---5---/-/-<---- v ^ v ^ | | S S |
 | | | | -->---6---/-/-->-- | | | | | | | U U |
 | v ^ | | / / 6 5 4 3 8 7 | | L L |
 | | | | | / / | | | | | | | | A A |
 | v ^ v ^ / / v ^ | | | | | | T T |
 +---------------------------------------------+ | E E |
 | | | | | | | | | | | | | | | |
 v ^ v ^ v ^ v ^ v ^ v ^ Original| | | |
 | | | | | | | | | | | | Packets | v ^ |
 +-----------------------+ +-----------------------+ | | |
 | | | | | | | | | | | |
 | | | | ---|----|-------<-------- | |
 | | | --->--------------->------>---- |
 | | | |
 | Link-Layer Protocols | | IPv6 Tunnel Link-Layer |
 +-----------------------+ +-----------------------------------+
 Fig.5 Packet Flow in the IPv6 Tunneling Protocol Engine on a Node
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 Note:
 In Fig.5, the Upper-Layer Protocols box represents transport
 protocols such as TCP, UDP, control protocols such as ICMP, routing
 protocols such as OSPF, and internet or lower-layer protocol being
 "tunneled" over IPv6, such as IPv4, IPX, etc. The Link-Layer
 Protocols box represents Ethernet, Token Ring, FDDI, PPP, X.25, Frame
 Relay, ATM, etc..., as well as internet layer "tunnels" such as IPv4
 tunnels.
 The IPv6 tunnel protocol engine acts as both an "upper-layer" and a
 "link-layer", each with a specific input and output as follows:
 (u.i) "tunnel upper-layer input" - consists of tunnel IPv6 packets
 that are going to be decapsulated. The tunnel packets are
 incoming through the IPv6 layer from:
 (u.i.1) a link-layer - (path #1, Fig.5)
 These are tunnel packets destined to this node and will
 undergo decapsulation.
 (u.i.2) a tunnel link-layer - (path #7, Fig.5)
 These are tunnel packets that underwent one or more
 decapsulations on this node, that is, the packets had
 one or more nested tunnel headers and one nested tunnel
 header was just discarded. This node is the exit-point
 of both an outer tunnel and one or more of its inner
 tunnels.
 For both above cases the resulting original packets are passed
 back to the IPv6 layer as "tunnel link-layer" output for
 further processing (see b.2).
 (u.o) "tunnel upper-layer output" - consists of tunnel IPv6 packets
 that are passed through the IPv6 layer down to:
 (u.o.1) a link-layer - (path #2, Fig.5)
 These packets underwent encapsulation and are sent
 towards the tunnel exit-point
 (u.o.2) a tunnel link-layer - (path #8, Fig.5)
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 These tunnel packets undergo nested encapsulation. This
 node is the entry-point node of both an outer tunnel
 and one or more of its inner tunnel.
 Implementation Note:
 The tunnel upper-layer input and output can be implemented similar
 to the input and output of the other upper-layer protocols.
 The tunnel link-layer input and output are as follows:
 (l.i) "tunnel link-layer input" - consists of original IPv6 packets
 that are going to be encapsulated.
 The original packets are incoming through the IPv6 layer from:
 (l.i.1) an upper-layer - (path #4, Fig.5)
 These are original packets originating on this node
 that undergo encapsulation. The original packet source
 and tunnel entry-point are the same node.
 (l.i.2) a link-layer - (path #6, Fig.5)
 These are original packets incoming from a different
 node that undergo encapsulation on this tunnel entry-
 point node.
 (l.i.3) a tunnel upper-layer - (path #8, Fig.5)
 These packets are tunnel packets that undergo nested
 encapsulation. This node is both the entry-point node
 of an outer tunnel and one or more of its inner
 tunnels.
 The resulting tunnel packets are passed as tunnel upper-layer
 output packets through the IPv6 layer (see u.o) down to:
 (l.o) "tunnel link-layer output" - consists of original IPv6 packets
 resulting from decapsulation. These packets are passed through
 the IPv6 layer to:
 (l.o.1) an upper-layer - (path #3, Fig.5)
 These original packets are destined to this node.
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 (l.o.2) a link-layer - (path #5, Fig.5)
 These original packets are destined to another node;
 they are transmitted on a link towards their
 destination.
 (l.o.3) a tunnel upper-layer - (path #7, Fig.5)
 These packets undergo another decapsulation; they were
 nested tunnel packets. This node is both the exit-
 point node of an outer tunnel and one or more inner
 tunnels.
 Implementation Note:
 The tunnel link-layer input and output can be implemented similar
 to the input and output of other link-layer protocols, for
 instance, associating an interface or pseudo-interface with the
 IPv6 tunnel.
 The selection of the "IPv6 tunnel link" over other links results
 from the packet forwarding decision taken based on the content of
 the node's routing table.
4. Nested Encapsulation
 Nested IPv6 encapsulation is the encapsulation of a tunnel packet.
 It takes place when a hop of an IPv6 tunnel is a tunnel. The tunnel
 containing a tunnel is called an outer tunnel. The tunnel contained
 in the outer tunnel is called an inner tunnel - see Fig.6. Inner
 tunnels and their outer tunnels are nested tunnels.
 The entry-point node of an "inner IPv6 tunnel" receives tunnel IPv6
 packets encapsulated by the "outer IPv6 tunnel" entry-point node. The
 "inner tunnel entry-point node" treats the receiving tunnel packets
 as original packets and performs encapsulation. The resulting
 packets are "tunnel packets" for the "inner IPv6 tunnel", and "nested
 tunnel packets" for the "outer IPv6 tunnel".
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 Outer Tunnel
 <------------------------------------->
 <--links--><-virtual link-><--links--->
 Inner Tunnel
 Outer Tunnel Outer Tunnel
 Entry-Point Exit-Point
 Node Node
 +-+ +-+ +-+ +-+ +-+ +-+
 | | | | | | | | | | | |
 | |->-//->-| |=>=//=>=| |**>**//**>**| |=>=//=>==| |->-//->-| |
 | | | | | | | | | | | |
 +-+ +-+ +-+ +-+ +-+ +-+
 Original Inner Tunnel Inner Tunnel Original
 Packet Entry-Point Exit-Point Packet
 Source Node Node Destination
 Node Node
 Fig.6. Nested Encapsulation
4.1 Limiting Nested Encapsulation
 A tunnel IPv6 packet size is limited to the maximum IPv6 datagram
 size [RFC 1883]. Each encapsulation adds to the size of a tunnel
 packet the size of the tunnel IPv6 headers. Consequently, the number
 of tunnel headers, and therefore, the number of nested
 encapsulations, and furthermore, the number of "inner IPv6 tunnels"
 that an "outer IPv6 tunnel" can have are limited by the maximum
 packet size.
 The increase in the size of a tunnel IPv6 packet due to nested
 encapsulations may require fragmentation [RFC-1883] - see section 7.
 Furthermore, each fragmentation, due to nested encapsulation, of an
 already fragmented tunnel packet results in a doubling of the number
 of fragments. Moreover, it is probable that once this fragmentation
 begins, each new nested encapsulation results in yet additional
 fragmentation. Therefore limiting nested encapsulation is
 recommended.
 The proposed mechanism for limiting excessive nested encapsulation is
 a "tunnel encapsulation limit", which is carried in an IPv6
 Destination Option header.
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4.1.1 Tunnel Encapsulation Limit
 The "Tunnel Encapsulation Limit" destination option is provided only
 by tunnel entry-point nodes, it is discarded only by tunnel exit-
 point nodes, and it is used to carry optional information [RFC-1883]
 that need be examined only by tunnel entry-point nodes.
 The "Tunnel Encapsulation Limit" destination option is defined as
 follows:
 Option Type Opt Data Len Opt Data Len
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0 0 1 0 0| 1 | Tun Encap Lim |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type value 4
 - the highest-order two bits - set to 00 -
 indicate "skip over this option if the option is
 not recognized".
 - the third-highest-order bit - set to 0 -
 indicates that the option data in this option does
 not change en route to the packet's destination
 [RFC-1883].
 Opt Data Len value 1 - the data portion of the Option is one
 byte long.
 Opt Data Value the Tunnel Encapsulation Limit value - 8-bit
 unsigned integer.
 To avoid excessive nested encapsulation, an IPv6 tunnel entry-point
 node may prepend to a packet undergoing encapsulation a "Tunnel
 Encapsulation Limit - Destination Option". The "OptData Value" field
 of the option is set to:
 (a) a pre-configured value - if the packet being encapsulated
 has no IPv6 destination options header or no "Tunnel
 Encapsulation Limit" option in such a header - see section
 6.6.
 (b) a value resulting from a value stored in the IPv6
 destination options header - if such a header exist and if
 it contains a "Tunnel Encapsulation Limit" option. The
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 "OptData Value" of the extant option is copied into the
 newly prepended "Tunnel Encapsulation Limit" option and
 then decremented by one.
 This is an exception to the rule of processing a
 destination options extension header in that, although the
 entry-point node is not a destination node, during
 encapsulation, the IPv6 tunneling protocol engine looks
 ahead, for an IPv6 destination header with a "Tunnel
 Encapsulation Limit" option immediately following the
 current IPv6 main header.
 If the Tunnel Encapsulation Limit is decremented to zero,
 the packet undergoing encapsulation is discarded. When the
 packet is discarded, a Parameter Problem ICMP message
 [RFC-1885] is returned to the packet originator, which is
 the previous tunnel entry-point. The message points to the
 Opt Data Value field within the Tunnel Encapsulation Limit
 destination header of the packet. The field pointed to has
 a value of one.
 Two cases of encapsulation that should be avoided are described
 below:
4.1.2 Loopback Encapsulation
 A particular case of encapsulation which must be avoided is the
 loopback encapsulation. Loopback encapsulation takes place when a
 tunnel IPv6 entry-point node encapsulates tunnel IPv6 packets
 originated from itself, and destined to itself. This can generate an
 infinite processing loop in the entry-point node.
 To avoid such a case, it is recommended that an implementation have a
 mechanism that checks and rejects the configuration of a tunnel in
 which both the entry-point and exit-point node addresses belong to
 the same node. It is also recommended that the encapsulating engine
 check for and reject the encapsulation of a packet that has the pair
 of tunnel entry-point and exit-point addresses identical with the
 pair of original packet source and final destination addresses.
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4.1.3 Routing-Loop Nested Encapsulation
 In the case of a forwarding path with multiple level nested tunnels,
 a routing-loop from an inner tunnel to an outer tunnel is
 particularly dangerous when packets from the inner tunnels reenter an
 outer tunnel from which they have not yet exited. In such a case, the
 nested encapsulation becomes a recursive encapsulation with the
 negative effects described in 4.1. Because each nested encapsulation
 adds a tunnel header with a new hop limit value, the IPv6 hop limit
 mechanism cannot control the number of times the packet reaches the
 outer tunnel entry-point node, and thus cannot control the number of
 recursive encapsulations.
 When the path of a packet from source to final destination includes
 tunnels, the maximum number of hops that the packet can traverse
 should be controlled by two mechanisms used together to avoid the
 negative effects of recursive encapsulation in routing loops:
 (a) the original packet hop limit.
 It is decremented at each forwarding operation performed on
 an original packet. This includes each encapsulation of the
 original packet. It does not include nested encapsulations
 of the original packet
 (b) the tunnel IPv6 packet encapsulation limit.
 It is decremented at each nested encapsulation of the
 packet.
 For a discussion of the excessive encapsulation risk factors in
 nested encapsulation see Appendix A.
5. Tunnel IPv6 Header
 The tunnel entry-point node fills out a tunnel IPv6 main header
 [RFC-1883] as follows:
 Version:
 value 6
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 Priority:
 Depending on the entry-point node tunnel configuration, the
 priority can be set to that of either the original packet or
 a pre-configured value - see section 6.3.
 Flow label:
 Depending on the entry-point node tunnel configuration, the
 flow label can be set to a pre-configured value. The typical
 value is zero - see section 6.4.
 Payload Length:
 The original packet length, plus the length of the
 encapsulating (prepended) IPv6 extension headers, if any.
 Next header:
 The next header value according to [RFC-1883] from the
 Assigned Numbers RFC [RFC-1700 or its succesors ].
 For example, if the original packet is an IPv6 packet, this
 is set to:
 - decimal value 41 (Assigned payload type number for
 IPv6) - if there are no tunnel extension headers.
 - value 0 (Assigned payload type number for IPv6 Hop by
 Hop Options header) - if a hop by hop options header
 immediately follows the tunnel IPv6 header.
 - decimal value 60 (Assigned payload type number for
 IPv6 Destination Options header) - if a Tunnel
 Encapsulation Limit destination option header
 immediately follows the tunnel IPv6 header.
 Hop limit:
 The tunnel IPv6 header hop limit is set to a pre-configured
 value - see section 6.3.
 The default value for hosts is the Neighbor Discovery
 advertised hop limit [RFC-1970]. The default value for
 routers is the default IPv6 Hop Limit value from the
 Assigned Numbers RFC (64 at the time of writing this
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 document).
 Source Address:
 An IPv6 address of the outgoing interface of the tunnel
 entry-point node. This address is configured as the tunnel
 entry-point node address - see section 6.1.
 Destination Address:
 An IPv6 address of the tunnel exit-point node. If the tunnel
 is configured as a free-exit tunnel, then the IPv6 address
 of the destination from the original IPv6 header - see
 section 6.2.
5.1 Tunnel IPv6 Extension Headers
 Depending on IPv6 node configuration parameters, a tunnel entry-point
 node may append to the tunnel IPv6 main header one or more IPv6
 extension headers, such as hop by hop, routing, or others.
 To limit the number of nested encapsulations of a packet, if it was
 configured to do so - see section 6.6 - a tunnel entry-point appends
 as the last tunnel extension header a Tunnel Encapsulation Limit
 destination option header with fields set as follows:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Next Header |Hdr Ext Len = 0| Opt Type = 4 |Opt Data Len=1 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Tun Encap Lim |PadN Opt Type=1|Opt Data Len=1 | 0 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Next Header:
 Identifies the type of the original packet header. For
 example, if the original packet is an IPv6 packet, the next
 header protocol value is set to decimal value 41 (Assigned
 payload type number for IPv6).
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 Hdr Ext Len:
 Length of the Tunnel Encapsulation Limit destination option
 header in 8-octet units, not including the first 8 octets.
 Set to value 0, if no other options are present in this
 destination options header.
 Option Type:
 value 4 - see section 4.1.1.
 Opt Data Len:
 value 1 - see section 4.1.1.
 Tun Encap Lim:
 8 bit unsigned integer - see section 4.1.1.
 Option Type:
 value 1 - PadN option, to align the header following this
 header.
 Opt Data Len:
 value 1 - one octet of option data.
 Option Data:
 value 0 - one zero-valued octet.
6. IPv6 Tunnel State Variables
 The IPv6 tunnel state variables, some of which are or may be
 configured on the tunnel entry-point node, are:
6.1 IPv6 Tunnel Entry-Point Node Address
 The tunnel entry-point node address is one of the valid IPv6 unicast
 addresses of the entry-point node - the validation of the address at
 tunnel configuration time is recommended.
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 The tunnel entry-point node address is copied to the source address
 field in the tunnel IPv6 header during packet encapsulation.
6.2 IPv6 Tunnel Exit-Point Node Address
 The tunnel exit-point node address is used as IPv6 destination
 address for the tunnel IPv6 header. The tunnel exit-point node
 address can be configured with a specific IPv6 address, in which case
 the tunnel is called a fixed-exit tunnel. Such a tunnel acts like a
 virtual point to point link between the entry-point node and exit-
 point node. Alternatively, a tunnel exit-point address can be
 configured with no specific address, in which case the tunnel is
 called a free-exit tunnel. Such a tunnel acts like a virtual point to
 point link between the entry-point node and an exit-point node
 identified by the destination address from the original packet
 header.
 The tunnel exit-point node address is copied to the destination
 address field in the tunnel IPv6 header during packet encapsulation.
 The configuration of the tunnel entry-point and exit-point addresses
 is not subject to IPv6 Autoconfiguration, or IPv6 Neighbor Discovery.
6.3 IPv6 Tunnel Hop Limit
 An IPv6 tunnel is modeled as a "single-hop virtual link" tunnel, in
 which the passing of the original packet through the tunnel is like
 the passing of the original packet over a one hop link, regardless of
 the number of hops in the IPv6 tunnel.
 The "single-hop" mechanism should be implemented by having the tunnel
 entry point node set a tunnel IPv6 header hop limit independently of
 the hop limit of the original header.
 The "single-hop" mechanism hides from the original IPv6 packets the
 number of IPv6 hops of the tunnel.
 It is recommended that the tunnel hop limit be configured with a
 value that ensures:
 (a) that tunnel IPv6 packets can reach the tunnel exit-point
 node
 (b) a quick expiration of the tunnel packet if a routing loop
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 occurs within the IPv6 tunnel.
 The tunnel hop limit default value for hosts is the IPv6 Neighbor
 Discovery advertised hop limit [RFC-1970]. The tunnel hop limit
 default value for routers is the default IPv6 Hop Limit value from
 the Assigned Numbers RFC (64 at the time of writing this document).
 The tunnel hop limit is copied into the hop limit field of the tunnel
 IPv6 header of each packet encapsulated by the tunnel entry-point
 node.
6.4 IPv6 Tunnel Packet Priority
 The IPv6 Tunnel Packet Priority indicates the value that a tunnel
 entry-point node sets in the priority field of a tunnel header. The
 default value is zero. The configured Packet Priority can also
 indicate whether the value of the priority field in the tunnel header
 is copied from the original header, or it is set to the pre-
 configured value.
6.5 IPv6 Tunnel Flow Label
 The IPv6 Tunnel Flow Label indicates the value that a tunnel entry-
 point node sets in the flow label of a tunnel header. The default
 value is zero.
6.6 IPv6 Tunnel Encapsulation Limit
 The Tunnel Encapsulation Limit value can indicate whether the entry-
 point node is configured to limit the number of encapsulations of
 tunnel packets originating on that node. The IPv6 Tunnel
 Encapsulation Limit is the maximum number of encapsulations permitted
 for packets undergoing encapsulation at that entry-point node.
 Recommended default value is 5. An entry-point node configured to
 limit the number of nested encapsulations prepends a Tunnel
 Encapsulation Limit destination options header to an original packet
 undergoing encapsulation - see section 4.1, and 4.1.1.
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6.7 IPv6 Tunnel MTU
 The tunnel MTU is set dynamically to the Path MTU between the tunnel
 entry-point and the tunnel exit-point nodes minus the size of the
 tunnel headers: the maximum size of a tunnel packet payload that can
 be sent through the tunnel without fragmentation [RFC-1883]. The
 tunnel entry-point node performs Path MTU discovery on the path
 between the tunnel entry-point and exit-point nodes [RFC-1981],
 [RFC-1885]. The tunnel MTU of a nested tunnel is the tunnel MTU of
 the outer tunnel minus the size of the tunnel headers.
 Although it should be able to send a tunnel IPv6 packet of any valid
 size, a tunnel entry-point node attempts to avoid the fragmentation
 of tunnel packets, by reporting to source nodes of original packets
 the MTU to be used in sizing original packets sent towards that
 tunnel entry-point node.
7. IPv6 Tunnel Packet Size Issues
 Prepending a tunnel header increases the size of a packet, therefore
 a tunnel packet resulting from the encapsulation of an IPv6 original
 packet may require fragmentation.
 A tunnel IPv6 packet resulting from the encapsulation of an original
 packet is considered an IPv6 packet originating from the tunnel
 entry-point node. Therefore, like any source of an IPv6 packet, a
 tunnel entry-point node must support fragmentation of tunnel IPv6
 packets.
 A tunnel intermediate node that forwards a tunnel packet to another
 node in the tunnel follows the general IPv6 rule that it must not
 fragment a packet undergoing forwarding.
 A tunnel exit-point node receiving tunnel packets at the end of the
 tunnel for decapsulation applies the strict left-to-right processing
 rules for extension headers. In the case of fragmentation headers,
 the fragments are reassembled into a tunnel packet before determining
 that an embedded IP packet is present.
 Note:
 A particular problem arises when the destination of a fragmented
 tunnel packet is an exit-point node identified by an anycast address.
 The problem, which is similar to that of original fragmented IPv6
 packets destined to nodes identified by an anycast address, consists
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 in the requirement that all the fragments of a packet must arrive to
 the same destination node, for that node to be able to perform a
 successful reassembly.
7.1 IPv6 Tunnel Packet Fragmentation
 Tunnel packets that exceed the tunnel MTU are candidates for
 fragmentation. The fragmentation of tunnel packets containing IPv6
 original packets is performed as follows:
 (a) if the original IPv6 packet size is larger than 576 octets,
 the entry-point node discards the packet and it returns an
 ICMPv6 "Packet Too Big" message to the source node of the
 original packet with the recommended MTU size field set to
 the maximum between 576, and the tunnel MTU, i.e. max(576,
 tunnel MTU). Note that the tunnel MTU is the Path MTU
 between the tunnel entry-point and the tunnel exit-point
 nodes minus the size of the tunnel headers. Also see
 section 6.7, and 8.2.
 (b) if the original IPv6 packet is equal or smaller than 576
 octets, the tunnel entry-point node encapsulates the
 original packet, and subsequently fragments the resulting
 IPv6 tunnel packet into IPv6 fragments that do not exceed
 the tunnel MTU.
7.2 IPv4 Tunnel Packet Fragmentation
 Tunnel packets that exceed the tunnel MTU are candidates for
 fragmentation. The fragmentation of tunnel packets containing IPv4
 original packets is performed as follows:
 (a) if in the original IPv4 packet header the Don't Fragment -
 DF - bit flag is SET, the entry-point node discards the
 packet and returns an ICMP message. The ICMP message has
 the type = "unreachable", the code = "datagram too big",
 and the recommended MTU size field set to the size of the
 tunnel MTU - see section 6.7, and 8.3.
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 (b) if in the original packet header the Don't Fragment - DF -
 bit flag is CLEAR, the tunnel entry-point node encapsulates
 the original packet, and subsequently fragments the
 resulting IPv6 tunnel packet into IPv6 fragments that do
 not exceed the tunnel MTU.
8. IPv6 Tunnel Error Processing and Reporting
 IPv6 Tunneling follows the general rule that an error detected during
 the processing of an IPv6 packet is reported through an ICMP message
 to the source of the packet.
 On a forwarding path that includes IPv6 tunnels, an error detected by
 a node that is not in any tunnel is directly reported to the source
 of the original IPv6 packet.
 An error detected by a node inside a tunnel is reported to the source
 of the tunnel packet, that is, the tunnel entry-point node. The ICMP
 message sent to the tunnel entry-point node has as ICMP payload the
 tunnel IPv6 packet that has the original packet as its payload.
 The cause of a packet error encountered inside a tunnel can be a
 problem with:
 (a) the tunnel header, or
 (b) the tunnel packet.
 Both tunnel header and tunnel packet problems are reported to the
 tunnel entry-point node.
 If a tunnel packet problem is a consequence of a problem with the
 original packet, which is the payload of the tunnel packet, then the
 problem is also reported to the source of the original packet.
 To report a problem detected inside the tunnel to the source of an
 original packet, the tunnel entry point node must relay the ICMP
 message received from inside the tunnel to the source of that
 original IPv6 packet.
 An example of the processing that can take place in the error
 reporting mechanism of a node is illustrated in Fig.7, and Fig.8:
 Fig.7 path #0 and Fig.8 (a) - The IPv6 tunnel entry-point receives an
 ICMP packet from inside the tunnel, marked Tunnel ICMPv6 Message in
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 Fig.7. The tunnel entry-point node IPv6 layer passes the received
 ICMP message to the ICMPv6 Input. The ICMPv6 Input, based on the ICMP
 type and code [RFC-1885] generates an internal "error code".
 Fig.7 path #1 - The internal error code, is passed with the "ICMPv6
 message payload" to the upper-layer protocol - in this case the IPv6
 tunnel upper-layer error input.
 +-------+ +-------+ +-----------------------+
 | Upper | | Upper | | Upper |
 | Layer | | Layer | | Layer |
 | Proto.| | Proto | | IPv6 Tunnel |
 | Error | | Error | | Error |
 | Input | | Input | | Input |
 | | | | | Decapsulate |
 | | | | | -->--ICMPv6--#2->-- |
 | | | | | | Payload | |
 +-------+ +-------+ +--|-----------------|--+
 | | | |
 ^ ^ ^ v
 | | | |
 --------------------#1-- -----Orig.Packet?--- - - - - - - - - -
 #1 #3 Int.Error Code, #5 |
Int.Error Code,^ v Source Address, v v
ICMPv6 Payload | IPv6 | Orig. Packet | IPv4 |
 +--------------+ +--------------+ +--------------+ + - - - - +
 | | | | | |
 | ICMP v6 | | ICMP v6 | | ICMP v4 | | |
 | Input | | Error Report | | Error Report |
 | - - - - +----+ - - - - | + - - - - + + - - - - +
 | | | |
 | IPv6 Layer | | IPv4 Layer | | |
 | | | |
 +----------------------------------+ +--------------+ + - - - - +
 | | |
 ^ V V
 #0 #4 #6
 | | |
 Tunnel ICMPv6 ICMPv6 ICMPv4
 Message Message Message
 | | |
 Fig.7 Error Reporting Flow in a Node (IPv6 Tunneling Protocol Engine)
 Fig.7 path #2 and Fig.8 (b) - The IPv6 tunnel error input
 decapsulates the tunnel IPv6 packet, which is the ICMPv6 message
 payload, obtaining the original packet, and thus the original headers
 and dispatches the "internal error code", the source address from the
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 original packet header, and the original packet, down to the error
 report block of the protocol identified by the Next Header field in
 the tunnel header immediately preceding the original packet in the
 ICMP message payload.
 From here the processing depends on the protocol of the original
 packet:
 (a) - for an IPv6 original packet
 Fig.7 path #3 and Fig.8 (c.1)- for an IPv6 original packet, the
 ICMPv6 error report builds an ICMP message of a type and code
 according to the "internal error code", containing the "original
 packet" as ICMP payload.
 Fig.7 path #4 and Fig.8 (d.1)- The ICMP message has the tunnel
 entry-point node address as source address, and the original packet
 source node address as destination address. The tunnel entry-point
 node sends the ICMP message to the source node of the original
 packet.
 (b) - for an IPv4 original packet
 Fig.7 path #5 and Fig.8 (c.2) - for an IPv4 original packet, the
 ICMPv4 error report builds an ICMP message of a type and code
 derived from the the "internal error code", containing the
 "original packet" as ICMP payload.
 Fig.7 path #6 and Fig.8 (d.2) - The ICMP message has the tunnel
 entry-point node IPv4 address as source address, and the original
 packet IPv4 source node address as destination address. The tunnel
 entry-point node sends the ICMP message to the source node of the
 original packet.
 A graphical description of the header processing taking place is the
 following:
 < Tunnel Packet >
 +--------+- - - - - -+--------+------------------------------//------+
 | IPv6 | IPv6 | ICMP | Tunnel |
(a)| | Extension | | IPv6 |
 | Header | Headers | Header | Packet in error |
 +--------+- - - - - -+--------+------------------------------//------+
 < Tunnel Headers > < Tunnel ICMP Message >
 < ICMPv6 Message Payload >
 |
 v
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 < Tunnel ICMP Message >
 < Tunnel IPv6 Packet in Error >
 +--------+ +---------+ +----------+--------//------+
 | ICMP | | Tunnel | | Original | Original |
(b) | | + | IPv6 | + | | Packet |
 | Header | | Headers | | Headers | Payload |
 +--------+ +---------+ +----------+--------//------+
 | <Original Packet in Error >
 ----------------- |
 | |
 --------------|---------------
 | |
 V V
 +---------+ +--------+ +-------------------//------+
 | New | | ICMP | | |
(c.1) | IPv6 | + | | + | Orig. Packet in Error |
 | Headers | | Header | | |
 +---------+ +--------+ +-------------------//------+
 |
 v
 +---------+--------+-------------------//------+
 | New | ICMP | Original |
(d.1) | IPv6 | | |
 | Headers | Header | Packet in Error |
 +---------+--------+-------------------//------+
 < New ICMP Message >
 or for an IPv4 original packet
 +---------+ +--------+ +-------------------//------+
 | New | | ICMP | | |
(c.2) | IPv4 | + | | + | Orig. Packet in Error |
 | Header | | Header | | |
 +---------+ +--------+ +-------------------//------+
 |
 v
 +---------+--------+-------------------//------+
 | New | ICMP | Original |
(d.2) | IPv4 | | |
 | Header | Header | Packet in Error |
 +---------+--------+-------------------//------+
 < New ICMP Message >
 Fig.8 ICMP Error Reporting and Processing
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8.1 Tunnel ICMP Messages
 The tunnel ICMP messages that are reported to the source of the
 original packet are:
 hop limit exceeded
 The tunnel has a misconfigured hop limit, or contains a
 routing loop, and packets do not reach the tunnel exit-
 point node. This problem is reported to the tunnel entry-
 point node, where the tunnel hop limit can be reconfigured
 to a higher value. The problem is further reported to the
 source of the original packet as described in section 8.2,
 or 8.3.
 unreachable node
 One of the nodes in the tunnel is not or is no longer
 reachable. This problem is reported to the tunnel entry-
 point node, which should be reconfigured with a valid and
 active path between the entry and exit-point of the tunnel.
 The problem is further reported to the source of the
 original packet as described in section 8.2, or 8.3.
 parameter problem
 A Parameter Problem ICMP message pointing to a valid Tunnel
 Encapsulation Limit Destination header with a Tun Encap Lim
 field value set to one is an indication that the tunnel
 packet exceeded the maximum number of encapsulations
 allowed. The problem is further reported to the source of
 the original packet as described in section 8.2, or 8.3.
 The above three problems detected inside the tunnel, which are a
 tunnel configuration and a tunnel topology problem, are reported to
 the source of the original IPv6 packet, as a tunnel generic
 "unreachable" problem caused by a "link problem" - see section 8.2
 and 8.3.
 packet too big
 The tunnel packet exceeds the tunnel Path MTU.
 The information carried by this type of ICMP message is
 used as follows:
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 - by a receiving tunnel entry-point node to set or adjust
 the tunnel MTU
 - by a sending tunnel entry-point node to indicate to the
 source of an original packet the MTU size that should be
 used in sending IPv6 packets towards the tunnel entry-point
 node.
8.2 ICMP Messages for IPv6 Original Packets
 The tunnel entry-point node builds the ICMP and IPv6 headers of the
 ICMP message that is sent to the source of the original packet as
 follows:
 IPv6 Fields:
 Source Address
 A valid unicast IPv6 address of the outgoing interface.
 Destination Address
 Copied from the Source Address field of the Original
 IPv6 header.
 ICMP Fields:
 For any of the following tunnel ICMP error messages:
 "hop limit exceeded"
 "unreachable node"
 "parameter problem" - pointing to a valid Tunnel Encapsulation
 Limit destination header with the Tun Encap Lim field set to a
 value one:
 Type 1 - unreachable node
 Code 3 - address unreachable
 For tunnel ICMP error message "packet too big":
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 Type 2 - packet too big
 Code 0
 MTU The MTU field from the tunnel ICMP message minus
 the length of the tunnel headers.
 According to the general rules described in 7.1, an ICMP "packet too
 big" message is sent to the source of the original packet only if the
 original packet size is larger than 576 octets.
8.3 ICMP Messages for IPv4 Original Packets
 The tunnel entry-point node builds the ICMP and IPv4 header of the
 ICMP message that is sent to the source of the original packet as
 follows:
 IPv4 Fields:
 Source Address
 A valid unicast IPv4 address of the outgoing interface.
 Destination Address
 Copied from the Source Address field of the Original
 IPv4 header.
 ICMP Fields:
 For any of the following tunnel ICMP error messages:
 "hop limit exceeded"
 "unreachable node"
 "parameter problem" - pointing to a valid Tunnel Enacpsulation
 Limit destination header with the Tun Encap Lim field set to a
 value one:
 Type 3 - destination unreachable
 Code 1 - host unreachable
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 For a tunnel ICMP error message "packet too big":
 Type 3 - destination unreachable
 Code 4 - datagram too big
 MTU The MTU field from the tunnel ICMP message minus
 the length of the tunnel headers.
 According to the general rules described in section 7.2, an ICMP
 "datagram too big" message is sent to the original IPv4 packet source
 node if the the original IPv4 header has the DF - don't fragment -
 bit flag SET.
8.4 ICMP Messages for Nested Tunnels Packets
 In case of an error uncovered with a nested tunnels packet, the inner
 tunnel entry-point, which receives the ICMP error message from the
 inner tunnel reporting node, relays the ICMP message to the outer
 tunnel entry-point following the mechanisms described in sections
 8.,8.1, 8.2, and 8.3. Further, the outer tunnel entry-point relays
 the ICMP message to the source of the original packet, following the
 same mechanisms.
9. Security Considerations
 An IPv6 tunnel can be secured, by securing the IPv6 path between the
 tunnel entry-point and exit-point node. The security architecture,
 mechanisms, and services are described in [RFC1825], [RFC1826], and
 [RFC1827]. A secure IPv6 tunnel may act as a gateway-to-gateway
 secure path as described in [RFC1825].
 For a secure IPv6 tunnel, in addition to the mechanisms described
 earlier in this document, the entry-point node of the tunnel performs
 security algorithms on the packet and prepends as part of the tunnel
 headers one or more security headers in conformance with [RFC1883],
 [RFC1825], and [RFC1826], or [RFC1827].
 The exit-point node of a secure IPv6 tunnel performs security
 algorithms and processes the tunnel security header[s] as part of the
 tunnel headers processing described earlier, and in conformance with
 [RFC1825], and [RFC1826], or [RFC1827]. The exit-point node discards
 the tunnel security header[s] with the rest of the tunnel headers
 after tunnel headers processing completion.
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 The degree of integrity, authentication, and confidentiality and the
 security processing performed on a tunnel packet at the entry-point
 and exit-point node of a secure IPv6 tunnel depend on the type of
 security header - authentication (AH) or encryption (ESP) - and
 parameters configured in the Security Association for the tunnel.
 There is no dependency or interaction between the security level and
 mechanisms applied to the tunnel packets and the security applied to
 the original packets which are the payloads of the tunnel packets. In
 case of nested tunnels, each inner tunnel may have its own set of
 security services, independently from those of the outer tunnels, or
 of those between the source and destination of the original packet.
10. Acknowledgments
 This document is partially derived from several discussions about
 IPv6 tunneling on the IPng Working Group Mailing List and from
 feedback from the IPng Working Group to an IPv6 presentation that
 focused on IPv6 tunneling at the 33rd IETF, in Stockholm, in July
 1995.
 Additionally, the following documents that focused on tunneling or
 encapsulation were helpful references: RFC 1933 (R. Gilligan, E.
 Nordmark), RFC 1241 (R. Woodburn, D. Mills), RFC 1326 (P. Tsuchiya),
 RFC 1701, RFC 1702 (S. Hanks, D. Farinacci, P. Traina), RFC 1853 (W.
 Simpson), as well as RFC 2003 (C. Perkins).
 Brian Carpenter, Richard Draves, Bob Hinden, Thomas Narten, Erik
 Nordmark (in alphabetical order) gave valuable reviewing comments and
 suggestions for the improvement of this document. Scott Bradner, Ross
 Callon, Dimitry Haskin, Paul Traina, and James Watt (in alphabetical
 order) shared their view or experience on matters of concern in this
 document. Judith Grossman provided a sample of her many years of
 editorial and writing experience as well as a good amount of probing
 technical questions.
11. References
 [RFC-1883] S. Deering, R. Hinden, "Internet Protocol Version 6
 Specification"
 [RFC-1885] A. Conta, and S. Deering "Internet Control Message
 Protocol for the Internet Protocol Version 6 (IPv6)"
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 [RFC-1970] T. Narten, E. Nordmark, W.Simpson "Neighbor Discovery for
 IP Version 6 (IPv6)"
 [RFC-1981] J. McCann, S. Deering, J. Mogul "Path MTU Discovery for IP
 Version 6 (IPv6)"
 [RFC-1825] R. Atkinson, "Security Architecture for the Internet
 Protocol"
 [RFC-1826] R. Atkinson, "IP Authentication Header"
 [RFC-1827] R. Atkinson, "IP Encapsulation Security Payload (ESP)"
 [RFC-1853] W. Simpson, "IP in IP Tunneling"
 [RFC-1700] J. Reynolds, J. Postel, "Assigned Numbers", 10/20/1994
Authors' Addresses:
 Alex Conta Stephen Deering
 Lucent Technologies Inc. Cisco Systems
 1300 Massaschussets Ave 170 West Tasman Dr
 Boxborough, MA 01719 San Jose, CA 95132-1706
 +1-508-263-3600/ext 535 +1-408-527-8213
 email: aconta@lucent.com email: deering@cisco.com
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Appendix A
A.1 Risk Factors in Nested Encapsulation
 Nested encapsulations of a packet become a recursive encapsulation if
 the packet reenters an outer tunnel before exiting it. The cases
 which present a high risk of recursive encapsulation are those in
 which a tunnel entry-point node cannot determine whether a packet
 that undergoes encapsulation reenters the tunnel before exiting it.
 Routing loops that cause tunnel packets to reenter a tunnel before
 exiting it are certainly the major cause of the problem. But since
 routing loops exist, and happen, it is important to understand and
 describe, the cases in which the risk for recursive encapsulation is
 higher.
 There are two significant elements that determine the risk factor of
 routing loop recursive encapsulation:
 (a) the type of tunnel,
 (b) the type of route to the tunnel exit-point, which
 determines the packet forwarding through the tunnel, that
 is, over the tunnel virtual-link.
A.1.1 Risk Factor in Nested Encapsulation - type of tunnel.
 The type of tunnels which were identified as a high risk factor for
 recursive encapsulation in routing loops are:
 "inner tunnels with identical exit-points".
 These tunnels can be:
 "fixed-end inner tunnels with different entry-points",
 or:
 "free-end inner tunnels with different entry-points"
 Note that free-end inner tunnels fall always into the category of
 identical exit-point tunnels.
 Since the source and destination of an original packet is the main
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 information used to decide whether to forward a packet through a
 tunnel or not, a recursive encapsulation can be avoided in case of a
 single tunnel (non-inner), by checking that the packet to be
 encapsulated is not originated on the entry-point node. This
 mechanism is suggested in [RFC-1853].
 However, this type of protection does not seem to work well in case
 of inner tunnels with different entry-points, and identical exit-
 points.
 Inner tunnels with different entry-points and identical exit-points
 introduce ambiguity in deciding whether to encapsulate a packet, when
 a packet encapsulated in an inner tunnel reaches the entry-point node
 of an outer tunnel by means of a routing loop. Because the source of
 the tunnel packet is the inner tunnel entry-point node which is
 different than the entry-point node of the outer tunnel, the source
 address checking (mentioned above) fails to detect an invalid
 encapsulation, and as a consequence the tunnel packet gets
 encapsulated at the outer tunnel each time it reaches it through the
 routing loop.
A.1.2 Risk Factor in Nested Encapsulation - type of route.
 The type of route to a tunnel exit-point node has been also
 identified as a high risk factor of recursive encapsulation in
 routing loops.
 One type of route to a tunnel exit-point node is a route to a
 specified destination node, that is, the destination is a valid
 specified IPv6 address (route to node). Such a route can be selected
 based on the longest match of an original packet destination address
 with the destination address stored in the tunnel entry-point node
 routing table entry for that route. The packet forwarded on such a
 route is first encapsulated and then forwarded towards the tunnel
 exit-point node.
 Another type of route to a tunnel exit-point node is a route to a
 specified prefix-net, that is, the destination is a valid specified
 IPv6 prefix (route to net). Such a route can be selected based on the
 longest path match of an original packet destination address with the
 prefix destination stored in the tunnel entry-point node routing
 table entry for that route. The packet forwarded on such a route is
 first encapsulated and then forwarded towards the tunnel exit-point
 node.
 And finally another type of route to a tunnel exit-point is a default
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 route, or a route to an unspecified destination. This route is
 selected when no-other match for the destination of the original
 packet has been found in the routing table. A tunnel that is the
 first hop of a default route is a "default tunnel".
 If the route to a tunnel exit-point is a route to node, the risk
 factor for recursive encapsulation is minimum.
 If the route to a tunnel exit-point is a route to net, the risk
 factor for recursive encapsulation is medium. There is a range of
 destination addresses that will match the prefix the route is
 associated with. If one or more inner tunnels with different tunnel
 entry-points have exit-point node addresses that match the route to
 net of an outer tunnel exit-point, then a recursive encapsulation may
 occur if a tunnel packet gets diverted from inside such an inner
 tunnel to the entry-point of the outer tunnel that has a route to its
 exit-point that matches the exit-point of an inner tunnel.
 If the route to a tunnel exit-point is a default route, the risk
 factor for recursive encapsulation is maximum. Packets are forwarded
 through a default tunnel for lack of a better route. In many
 situations, forwarding through a default tunnel can happen for a wide
 range of destination addresses which at the maximum extent is the
 entire Internet minus the node's link. As consequence, it is likely
 that in a routing loop case, if a tunnel packet gets diverted from an
 inner tunnel to an outer tunnel entry-point in which the tunnel is a
 default tunnel, the packet will be once more encapsulated, because
 the default routing mechanism will not be able to discern
 differently, based on the destination.
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