rfc1241.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

For Data Messages, the Encapsulation Protocol Header is followed by the
Clear Datagram.  For Error Messages, the header is followed by the ICMP
message being forwarded along a flow.

B. Encapsulation and Existing IP Mechanisms

   This section discusses in detail the effect of this encapsulation
   protocol upon the existing mechanisms available with IP and some the
   possible effects of IP mechanisms upon this protocol.  Specifically
   these are Fragmentation and ICMP messages.

B.1 Fragmentation and Maximum Transmission Unit

   An immediate concern of using an encapsulation mechanism is that of
   restrictions based upon MTU size.  The source of a Clear Datagram is
   going to generate packets consistent with MTU of the interface over
   which datagram is transmitted.  If these packets reach an
   Encapsulator and are encapsulated, they may be fragmented if they are
   larger than the MTU of the Encapsulator, even though the physical
   interfaces of the source and Encapsulator may have the same MTU.
   Because the Encapsulated Datagram is sent to the Decapsulator using
   IP, there is no problem in allowing IP to perform fragmentation and
   reassembly.  However, fragmentation is known to be inefficient and is
   generally avoided.  Because a new header is being prepended to the
   Clear Datagram by the encapsulation process, the likelihood of
   fragmentation occurring is increased.  If the Encapsulator decides to
   disallow fragmentation through the Encapsulation Space, it must send
   an ICMP message back to the source.  This means that the MTU of the
   interface in the encapsulation space is effectively smaller than that
   of the physical MTU of the interface.

   Fragmentation by intermediate User Space Gateways introduces another



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   problem.  Fragmentation occurs at the IP level.  If a TCP protocol is
   in use and fragmentation occurs, the TCP header is contained in the
   first fragment, but not the following fragments.  [3] If these
   fragments are forwarded by an Encapsulator, discrimination of the
   Clear Header for a given flow will only be able to occur on the IP
   header portion of the Clear Header.  If discrimination is attempted
   on the TCP portion of the header, then only the first fragment will
   be matched, while remaining fragments will not.

B.2 ICMP Messages

   The most controversial aspect of encapsulation is the handling of
   ICMP messages. [1] Because the Encapsulation Header contains the
   source address of the Encapsulator in the Encapsulation Space, ICMP
   messages which occur within the Encapsulation Space will be sent back
   to the Encapsulator.  Once the Encapsulator receives the ICMP
   message, the question is what should the next action be.  Since the
   original source of the Clear Datagram knows nothing about the
   Encapsulation Space, it does not make sense to forward an ICMP
   message on to it and ICMP message are not supposed to beget ICMP
   messages.  Yet not sending the original source something may break
   some important mechanisms.

   In addition to deciding what to forward to the source of the Clear
   Datagram, there is the problem of possibly not having enough
   information to send anything at all back to the source.  An ICMP
   message returns the header of the offending message and the first
   eight octets of the data after the header.  For the case of the
   encapsulation protocol, this translates to the IP portion of the
   Encapsulation Header, the first eight octets of the Encapsulation
   Protocol Header, and nothing else.  The contents of the Clear
   Datagram are completely lost.  Therefore, for the Encapsulator to
   send an ICMP message back to the source it has to reconstruct the
   Clear Header.  However, it is essentially impossible to reproduce the
   exact header.

   For the purpose of this specification, the Flow ID has been assumed
   to be a unique one way mapping from a Clear Header.  There is no
   guarantee that the Flow ID could be used to map back to the Clear
   Header, since several headers potentially map to the same flow.  With
   there being no effective way to regenerate the original datagram,
   some compromises must be examined.

   For each of the possible ICMP messages, the alternatives and impact
   will be assessed.  There are three categories of ICMP message
   involved.  The first is those ICMP messages which are not applicable
   in the context of Encapsulation.  These are: Echo/Echo Reply and
   Timestamp/Timestamp Reply.



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   The second category are those ICMP messages which concern mechanisms
   local to the encapsulation domain.  These are messages which would
   not make sense to the original source if it did receive them.  In
   these cases the encapsulator will have to decide what to do, but no
   ICMP message need be sent back to the original source.  The datagram
   will simply be lost, IP is not meant to be a reliable protocol.
   Subsequent messages received for encapsulation may cause the
   encapsulator to generate ICMP Destination Unreachable messages back
   to the original source if the encapsulator can no longer send
   messages to the destination decapsulator.  This requires that ICMP
   messages inside the encapsulation domain affect the mapping from the
   Flow ID.  ICMP messages in the second category are: Parameter
   Problem, Redirect, Destination Unreachable, Time Exceeded.

   Finally there is one ICMP message which has direct bearing on the
   operation of the original source of datagrams destined for
   encapsulation, the ICMP Source Quench message.  The only possible
   mechanism available to the Encapsulator to handle this message is for
   the source quench message set a flag for the offending Flow ID such
   that subsequent messages that map the Flow cause the generation of a
   source quench back to the original source before the datagram is
   encapsulated.

   This last mechanism may be a solution for the more general problem.
   The rule of thumb could be that when an ICMP message is received for
   a given flow, then flag the Flow so that then next message
   encapsulated will cause the next message encapsulated on that flow to
   force an ICMP message to the source.  After the ICMP message is sent
   to the source, the mechanism could be reset.  This would effectively
   cause every other packet to receive an ICMP message if there were a
   persistent problem.  This mechanism is probably only safe for
   Unreachable messages and Source Quench.

C. Reception of Clear Datagrams

   In order to use the encapsulation protocol a modification is required
   to IP forwarding.  There must be some way for the IP module in a
   system to pass Clear Datagrams to the encapsulation protocol.  A
   suggested means of doing this is to make an addition to a system's
   routing table structures.  A flag could be added to a route that
   tells the forwarding function to use encapsulation.  Note that the
   default route could also be set to use encapsulation.

   With this mechanism in place, a system's IP forwarding mechanism
   would examine its routing tables to try and match the IP destination
   to a specific route.  If a route was found, it would be then checked
   to see if encapsulation should be used.  If not the packet would be
   handled normally.  If encapsulation was turned on for the route, then



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   the datagram would be sent to encapsulation for forwarding.

   In addition  to snagging packets as they are forwarded, something
   must be  done at  the last  Decapsulator on  a given flow so that
   packets that  are decapsulated  are properly  dumped into  the IP
   module for  delivery.   Because the packets are encapsulated just
   before forwarding,  it should be a simple matter for decapsulated
   datagrams to be injected into the output portion of IP.  However, the
   source  address in  the Clear  Header must  not change.   The address
   must  remain the address of the source in the source User Space and
   not be overwritten with that of the Decapsulator.

D. Construction of Virtual Networks with Encapsulation

   Because of the modification to the routing table to permit
   encapsulation, it becomes possible to specify a virtual interface
   whose sole purpose is encapsulation.  Using this mechanism, it would
   become possible to link topologically distant entities with Flows.
   This would allow the construction of a Virtual Network which would
   overlay the actual routing topology.  An example of such a virtual
   network is shown in Fig. 4.






























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                                      ++++++  Virtual Network A
                                      ******  Virtual Network B
                                           #  Encapsulator/Decapsulator
                                      ------  Common Routing Space

           ------------                     ------------
          /            \                   /            \
         /      +++ #   \                 /              \
        |  # +++    +    |               |    # ***** #   |
        |  +        +    |               |    *       *   |
        |  +       +     |               |     *     *    |
        |   +      +     |               |      *   *     |
        |   # ++++ # +   |               |       * *      |
         \            + /  -------------  \       # **   /  ---------
          \           + # ++            \ # ******   *** # **        \
           ------------  /  +++          *  ------------  /  ***      \
                        |      #        * |              |      # *** #|
                        |      +      **  |              |      *     *|
                        |      +     #    |              |     *    ** |
                        |      + ++++ *   |              |    *    *   |
                        |       #+     *  |              |   *    *    |
           ------------  \  ++++        */  ------------  \ *    #     /
          /            \ # +             # **           * # *****     /
         /              +  -------------  /  # ****** # *\   --------
        |   # +++++++   +|               |   *        *   |
        |   +        + + |               |   *         *  |
        |    +         # |               |   *          * |
        |    +       ++  |               |   *          # |
        |    # ++++++    |               |   * *********  |
         \              /                 \   #          /
          \            /                   \            /
           ------------                     ------------


                       Fig. 4.  Virtual Networks Example

   Each Encapsulator shown has an virtual interface on one of the
   virtual networks.  The lines represent individual links in the flows
   that connect each member of the virtual network.  Note that new links
   could be added between any points as long as the two entities are
   visible to each other in a common Encapsulation Space.  The routing
   within the virtual network would be handled by the encapsulation
   mechanism.  The programming of the routing tables could be a variant
   of any of the currently existing routing protocols, an encapsulated
   OSPF for example.

   With this in mind, it would be possible to have special encapsulation
   gateways with virtual interfaces on two virtual networks to form an



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   entire virtual internet.  This is the role of the Encapsulators
   joining Virtual Network A and Virtual Network B.

E. Encapsulation and OSI

   It is intended that the encapsulation mechanism described in the memo
   be extensible to other environments outside of the Internet.  It
   should be possible to encapsulate many different protocols within IP
   and IP within many other protocols.

   The key concepts defined in this memo are the mapping of a header to
   a Flow ID and the mapping of fields in the original header to the
   encapsulating header.  Special mappings between protocols would have
   to be defined, i.e. for the QoS bits, and some sort of translation of
   meanings carefully crafted, but it would be possible, none the less.

F. Security Considerations

   No means of authentication or integrity checking is specifically
   defined for this protocol apart from the checksum for the header
   information.  However for authentication or integrity checking to be
   used with this protocol, it is suggested that the authentication
   information be appended to the Encapsulated Datagram.  Information
   regarding the type of authentication or integrity check in use would
   have to be included in the flow management protocol which is used to
   distribute the flow information.

G. Authors' Addresses

   Robert A. Woodburn
   SAIC
   8619 Westwood Center Drive
   Vienna, VA  22182

   Phone:  [phone] or [phone]
   EMail:  [e-mail]


   David L. Mills
   Electrical Engineering Department
   University of Delaware
   Newark, DE  19716

   Phone:  [phone]
   EMail:  [e-mail]






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