rfc2473.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Conta & Deering             Standards Track                     [Page 6]

RFC 2473            Generic Packet Tunneling in IPv6       December 1998


   Tunnel extension headers should appear in the order recommended by
   the specifications that define the extension headers, such as [IPv6-
   Spec].

   A source of original packets and a tunnel entry-point that
   encapsulates those packets can be the same node.

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.



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RFC 2473            Generic Packet Tunneling in IPv6       December 1998


   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.

   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:

   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).




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RFC 2473            Generic Packet Tunneling in IPv6       December 1998


      +-----------------------+   +-----------------------------------+
      | 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

   (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)

                 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.



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RFC 2473            Generic Packet Tunneling in IPv6       December 1998


   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 the entry-point node of
                 both 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.

         (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.




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RFC 2473            Generic Packet Tunneling in IPv6       December 1998


         (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".



















Conta & Deering             Standards Track                    [Page 11]

RFC 2473            Generic Packet Tunneling in IPv6       December 1998


                 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 is limited to the maximum IPv6 packet size
   [IPv6-Spec].  Each encapsulation adds to the size of an encapsulated
   packet the size of the tunnel IPv6 headers. Consequently, the number
   of tunnel headers, and therefore, the number of nested encapsulations
   is limited by the maximum packet size.  However this limit is so
   large (more than 1600 encapsulations for an original packet of
   minimum size) that it is not an effective limit in most cases.

   The increase in the size of a tunnel IPv6 packet due to nested
   encapsulations may require fragmentation [IPv6-Spec] at a tunnel
   entry point - 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" option, which is carried in an IPv6
   Destination Options extension header accompanying an encapsulating
   IPv6 header.