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rfc:rfc1241

Network Working Group R. Woodburn Request for Comments: 1241 SAIC

                                                             D. Mills
                                               University of Delaware
                                                            July 1991
          A Scheme for an Internet Encapsulation Protocol:
                             Version 1

1. Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  Discussion and suggestions for improvement are requested.
 Please refer to the current edition of the "IAB Official Protocol
 Standards" for the standardization state and status of this protocol.
 Distribution of this memo is unlimited.

2. Glossary

 Clear Datagram -
   The unmodified IP datagram in the User Space before
   Encapsulation.
 Clear Header -
   The header portion of the Clear Datagram before
   Encapsulation.  This header includes the IP header and
   possibly part or all of the next layer protocol header,
   i.e., the TCP header.
 Decapsulation -
   The stripping of the Encapsulation Header and forwarding
   of the Clear Datagram by the Decapsulator.
 Decapsulator -
   The entity responsible for receiving an Encapsulated
   Datagram, decapsulating it, and delivering it to the
   destination User Space.  Delivery may be direct, or via
   Encapsulation.  A Decapsulator may be a host or a gateway.
 Encapsulated Datagram -
   The datagram consisting of a Clear Datagram prepended with
   an Encapsulation Header.
 Encapsulation -
   The process of mapping a Clear Datagram to the
   Encapsulation Space, prepending an Encapsulation Header to
   the Clear Datagram and routing the Encapsulated Datagram

Woodburn & Mills [Page 1] RFC 1241 Internet Encapsulation July 1991

   to a Decapsulator.
 Encapsulation Header -
   The header for the Encapsulation Protocol prepended to the
   Clear Datagram during Encapsulation.  This header consists
   of an IP header followed by an Encapsulation Protocol
   Header.
 Encapsulation Protocol Header -
   The Encapsulation Protocol specific portion of the
   Encapsulation Header.
 Encapsulation Space -
   The address and routing space within which the
   Encapsulators and Decapsulators reside.  Routing within
   this space is accomplished via Flows.  Encapsulation
   Spaces do not overlap, that is, the address of any
   Encapsulator or Decapsulator is unique for all
   Encapsulation Spaces.
 Encapsulator -
   The entity responsible for mapping a given User Space
   datagram to the Encapsulation Space, encapsulating the
   datagram, and forwarding the Encapsulated Datagram to a
   Decapsulator.  An Encapsulator may be a host or a gateway.
 Flow -
   Also called a "tunnel."  A flow is the end-to-end path in
   the Encapsulation Space over which Encapsulated Datagrams
   travel.  There may be several Encapsulator/Decapsulator
   pairs along a given flow.  Note that a Flow does not
   denote what User Space gateways are traversed along the
   path.
 Flow ID -
   A 32-bit identifier which uniquely distinguishes a flow in
   a given Encapsulator or Decapsulator.  Flow IDs are
   specific to a single Encapsulator/Decapsulator Entity and
   are not global quantities.
 Mapping Function -
   This is the function of mapping a Clear Header to a
   particular Flow.  All encapsulators along a given Flow are
   required to map a given Clear Header to the same Flow.
 User Address -
   The address or identifier uniquely identifying an entity
   within a User Space.

Woodburn & Mills [Page 2] RFC 1241 Internet Encapsulation July 1991

 Source Route -
   A complete end-to-end route which is computed at the
   source and enumerates transit gateways.
 User Space -
   The address and routing space within which the users
   reside.  Routing within this space provides reachability
   between all address pairs within the space.  User Spaces
   do not overlap, that is, a given User Address is unique in
   all User Spaces.

3. Background

 For several years researchers in the Internet community have needed a
 means of "tunneling" between networks.  A tunnel is essentially a
 Source Route that circumvents conventional routing mechanisms.
 Tunnels provide the means to bypass routing failures, avoid broken
 gateways and routing domains, or establish deterministic paths for
 experimentation.
 There are several means of accomplishing tunneling.  In the past,
 tunneling has been accomplished through source routing options in the
 IP header which allow gateways along a given path to be enumerated.
 The disadvantage of source routing in the IP header is that it
 requires the source to know something about the networks traversed to
 reach the destination.  The source must then modify outgoing packets
 to reflect the source route.  Current routing implementations
 generally don't support source routes in their routing tables as a
 means of reaching an IP address, nor do current routing protocols.
 Another means of tunneling would be to develop a new IP option.  This
 option field would be part of a separate IP header that could be
 prepended to an IP datagram.  The IP option would indicate
 information about the original datagram.  This tunneling option has
 the disadvantage of significantly modifying existing IP
 implementations to handle a new IP option.  It also would be less
 flexible in permitting the tunneling of other protocols, such as ISO
 protocols, through an IP environment.  An even less palatable
 alternative would be to replace IP with a new networking protocol or
 a new version of IP with tunneling built in as part of its
 functionality.
 A final alternative is to create a new IP encapsulation protocol
 which uses the current IP header format.  By using encapsulation, a
 destination can be reached transparently without the source having to
 know topology specifics.  Virtual networks can be created by tying
 otherwise unconnected machines together with flows through an
 encapsulation space.

Woodburn & Mills [Page 3] RFC 1241 Internet Encapsulation July 1991

                                             ++++++  Clear Datagram
                                             ******  Encapsulated
     Datagram
                                                  #
     Encapsulator/Decapsulator
                                                  &  User Space Host
         User Space A                        User Space C
  1. ————- ———–

/ \ / \

      /                \                /             \
     |                  |              |               |
     |     &            |              |               |
     |     +   +++++    |              |      *****    |
     |     +++++   +    |              |      *   *    |
     |             +    |              |  *****   *    |
      \            +   /  -----------  \ *       *    /  ----------
       \           ++> # *         **> # *        ***> # ++++      \
        --------------  / *        *  \  ------------  /   +        \
                       |  *        *   |              |    +         |
                       |  *        *   |              |    +         |
                       |  *****    *   |              |    +++++++   |
                       |      *****    |              |          V   |
                       |               |              |          &   |
                        \             /                \             /
                         \           /                  \           /
                          -----------                    ----------
                         Encapsulation                      User
                            Space B                        Space D
                Fig. 1.  Encapsulation Architectural Model
 Up until now, there has been no standard for an encapsulation
 protocol.  This RFC provides a means of performing encapsulation in
 the Internet environment.

4. Architecture and Approach

 The architecture for encapsulation is based on two entities -- an
 Encapsulator and a Decapsulator.  These entities and the associated
 spaces are shown in Fig. 1.
 Encapsulators and Decapsulators have addresses in the User Spaces to
 which they belong, as well as addresses in the Encapsulation Spaces
 to which they belong. An encapsulator will receive a Clear Datagram

Woodburn & Mills [Page 4] RFC 1241 Internet Encapsulation July 1991

 from its User Space, and after determining that encapsulation should
 be used, perform a mapping function which translates the User Space
 information in the Clear Header to an Encapsulation Header.  This
 Encapsulation Header is then prepended to the Clear Datagram to form
 the Encapsulated Datagram, as in Fig 2.  It is desirable that the
 encapsulation process be transparent to entities in the User Space.
 Only the Encapsulator need know that encapsulation is occurring.
       +---------------+-----------------+--------+----------------+
       | Encapsulating |  Encapsulation  | Clear  |  Remainder of  |
       |   IP Header   | Protocol Header | Header | Clear Datagram |
       +---------------+-----------------+--------+----------------+
       |                                 |                         |
       |        Encapsulation Header     |      Clear Datagram     |
       |                                 |                         |
               Fig. 2.  Example of an Encapsulated Datagram
 The Encapsulator forwards the datagram to a Decapsulator whose
 identity is determined at the time of encapsulation.  The
 Decapsulator receives the Encapsulated Datagram and removes the
 Encapsulation Header and treats the Clear Datagram as if it were
 received locally.  The requirement for the address of the
 Decapsulator is that it be reachable from the Encapsulator's
 Encapsulation Space address.

5. Generation of the Encapsulation Header

 The contents of the Encapsulation Header are generated by performing
 a mapping function from the Clear Header to the contents of the
 Encapsulation Header.  This mapping function could take many forms,
 but the end result should be the same.  The following paragraphs
 describe one method of performing the mapping.  The process is
 illustrated in Fig. 3.
 In the first part of the mapping function, the Clear Header is
 matched with stored headers and masks to determine a Flow ID.  This
 is essentially a "mask-and-match" table look up, where the lookup
 table holds three entries, a Clear Header, a header mask, and a
 corresponding Flow ID.  The mask can be used for allowing a range of
 source and destination addresses to map to a given flow.  Other
 fields, such as the IP TOS bits or even the TCP source or destination
 port addresses could also be used to discriminate between Flows.
 This flexibility allows many possibilities for using the mapping
 function.  Not only can a given network be associated with a
 particular flow, but even a particular TCP protocol or connection

Woodburn & Mills [Page 5] RFC 1241 Internet Encapsulation July 1991

 could be distinguished from another.
 How the lookup table is built and maintained is not part of this
 protocol.  It is assumed that it is managed by some higher layer
 entity.  It would be sufficient to configure the tables from ascii
 text files if necessary.
                                              +--------+
                                              |        |
                                           +->| Encap. |--+
                                           |  | Info.  |  |
                 +-------+                 |  | Table  |  |
                 | Mask  |   +---------+   |  |        |  |
     Clear --+-->|  &    |-->| Flow ID |---+  |        |  |
     Header  |   | Match |   +---------+      +--------+  |
             |   +-------+                                |
             |                                            +-->  Encap
             +----------------------------------------------->  Header
              Fig. 3.  Generation of the Encapsulation Header
 The Flow IDs are managed at a higher layer as well.  An example of
 how Flow IDs can be managed is found in the Setup protocol of the
 Inter-Domain Policy Sensitive Routing Protocol (IDPR). [4] The upper
 layer protocol would be responsible for maintaining information not
 carried in the encapsulation protocol related to the flow.  This
 could include the information necessary to construct the
 Encapsulation Header (described below) as well as information such as
 the type of data being encapsulated (currently only IP is defined),
 and the type of authentication used if any.  Note that IDPR Setup
 requires the use of a longer Flow ID which is unique for the entire
 universe of Encapsulators and is the same at every Encapsulator.
 The Flow ID that results from the mapping of a Clear Header is a 32
 bit quantity and identifies the Flow as it is seen by the
 Encapsulator.  If a Clear Datagram must be encapsulated and
 decapsulated several times in order reach the destination, the Flow
 ID may be different at each Encapsulator, but need not be.  The Flow
 ID acts as an index into a table of Encapsulation Header information
 that is used to build the Encapsulation Header.  Note that the
 decision to make the Flow ID local to the Encapsulator is due to the
 difficulty in choosing and maintaining globally unique identifiers.
 The intermediate step of using a Flow ID entirely optional.  The
 important requirement is that all Encapsulators along a Flow map the
 same Clear Header to the same Flow (which could be identified by
 different identifiers along the way).  However, by allowing for a

Woodburn & Mills [Page 6] RFC 1241 Internet Encapsulation July 1991

 Flow ID in the protocol, a more efficient implementation of the
 mapping function becomes possible.  This is discussed in more detail
 when we consider the Decapsulator.
 The following information is required to construct the Encapsulation
 Header:
 Flow ID -
   This is the key for this table of information and
   represents the Flow ID relative to the current
   Encapsulator.
 Decapsulator Address -
   The IP address of the Decapsulator in the Encapsulation
   Space must be known to build the IP portion of the
   Encapsulation Header.
 Decapsulator's Flow ID -
   The Flow ID, if any, for the Flow as seen by the
   Decapsulator must be known.
 Previous Encapsulator's Address -
   If this is not the first Encapsulator along the Flow, the
   previous Encapsulator's address must be known for error
   reporting.
 Previous Encapsulator's Flow ID -
   In addition to the previous Encapsulator's address, the
   Flow ID of the Flow relative to the previous Encapsulator
   must be known.
 The Encapsulation Header consists of an IP Header as well as an
 Encapsulation Protocol Header.  The two pieces of information
 required for the Encapsulation Protocol Header which must be
 determined at the time of encapsulation are the protocol which is
 being encapsulated and the Flow ID to send to the Decapsulator.  The
 generation of the IP header is more complicated.
 There are  two possible ways each field in the Clear Header could
 related to the new IP header.
 Copy -
   Copy the existing field from the Clear Header to the IP
   header in the Encapsulation Header.
 Ignore -
   The field may or may not have existed in the Clear Header,
   but does not apply to the new IP header.

Woodburn & Mills [Page 7] RFC 1241 Internet Encapsulation July 1991

 The IP header has a fixed portion and a variable portion, the options
 list.  A summary of all possible IP fields and the relation to the
 Clear Header follows in Table 1. [2]
 Note that most of the fields in the Clear Header are simply ignored.
 Fields such as the Header Length in the Clear Header have no effect
 on the Header Length of the new IP header.  The fields which are more
 interesting and require some thought are now discussed.
 The Quality of Service bits should be copied from the Clear Header to
 the new IP header.  This is in keeping with the transparency
 principle that if the User Space was providing a given service, then
 the Encapsulation Space must provide the same service.
 The More Fragments bit and Fragment Offset should not be copied,
 since the datagram being built is a complete datagram, regardless of
 the status of the encapsulated datagram.  If the completed datagram
 is too large for the interface, it will be fragmented for
 transmission to the decapsulator by the normal IP fragmentation
 mechanism.
 The Don't Fragment bit should not be copied into the Encapsulation
 Header.  The transparency principle would again be violated.  It
 should be up to the Encapsulator to decide whether fragmentation
 should be allowed across the Encapsulation Space.  If it is decided
 that the DF bit should be used, then ICMP message would be returned
 if the Encapsulated Datagram required fragmentation across the
 Encapsulation Space The mechanism for returning an ICMP message to
 the source in the User space will have to be modified, however, and
 this is discussed in the Appendix B.
 Regarding the Time To Live (TTL) field, the easiest thing to do is to
 ignore the TTL from the Clear Header.  If this field were copied from
 the Clear Header to the new IP header, the packet life might be
 prematurely exceeded during transit in the Encapsulation Space.  This
 breaks the transparency rule of encapsulation as seen from the User
 Space.  The TTL of the Clear Header is decremented before
 encapsulation by the IP forwarding function, so there is no chance of
 a packet looping forever if the links of a Flow form a loop.

Woodburn & Mills [Page 8] RFC 1241 Internet Encapsulation July 1991

                        +---------------------+---------+
                        |        Field        | Mapping |
                        +---------------------+---------+
                        | Version             | Ignore  |
                        | Header Length       | Ignore  |
                        | Precedence          | Copy    |
                        | QoS bits            | Copy    |
                        | Total Length        | Ignore  |
                        | Identification      | Ignore  |
                        | Don't Fragment Bit  | Ignore  |
                        | More Fragments Bit  | Ignore  |
                        | Fragment Offset     | Ignore  |
                        | Time to Live        | Ignore  |
                        | Protocol            | Ignore  |
                        | Header Checksum     | Ignore  |
                        | Source Address      | Ignore  |
                        | Destination Address | Ignore  |
                        | End of Option List  | Ignore  |
                        | NOP Option          | Ignore  |
                        | Security Option     | Copy    |
                        | LSR Option          | Ignore  |
                        | SSR Option          | Ignore  |
                        | RR Option           | Ignore  |
                        | Stream ID Option    | Ignore  |
                        | Timestamp Option    | Ignore  |
                        +---------------------+---------+
                     Table 1.  Summary of IP Header Mappings
 The protocol field for the new IP header should be filled with the
 protocol number of the encapsulation protocol.
 The source address in the new IP header becomes the IP address of the
 Encapsulator in the Encapsulation Domain.  The destination address
 becomes the IP address of the Decapsulator as found in the
 encapsulation table.
 IP Options are generally not copied because most don't make sense in
 the context of the Encapsulation Space, as the transparency principle
 would indicate.  The security option is probably the one option that
 should get copied for the same reason QOS and precedence fields are
 copied, the Encapsulation Space must provide the expected service.
 Timestamp, Loose Source Route, Strict Source Route, and Record Route
 are not copied during encapsulation.

6. Decapsulation

 In the ideal situation, a Decapsulator receives an Encapsulated

Woodburn & Mills [Page 9] RFC 1241 Internet Encapsulation July 1991

 Datagram, strips off the Encapsulation Header and sends the Clear
 Datagram back into IP so that it is forwarded from that point.
 However, if the Clear Datagram has not reached the destination User
 Space, it must again be encapsulated to move it close to the
 destination User Space.  In this latter case the Decapsulator would
 become an Encapsulator and would perform the same calculation to
 generate the Encapsulation Header as did the previous Encapsulator.
 In order to make this process more efficient, the use of Flow IDs
 have been incorporated into the protocol.
 When Flow IDs are used, the Flow ID received in the Encapsulation
 Header corresponds to a stored Flow ID in the Decapsulator.  At this
 point the Decapsulator has the option of bypassing the mask and match
 operation on the Clear Header.  The received Flow ID can be used to
 point directly into the local Encapsulator tables for the
 construction of the next Encapsulation Header.  If the Flow ID is
 unknown, an error message is sent back to the previous Encapsulator
 to that effect and a signal is sent to upper layer entity managing
 the encapsulation tables.
 Because the normal IP forwarding mechanism is being bypassed when
 Flow IDs are used, certain mechanisms normally handled by IP must be
 taken care of by the Decapsulator before encapsulation.  The
 Decapsulator must decrement the TTL before the next encapsulation
 occurs.  If a Time Exceeded error occurs, then an ICMP message is
 sent to the source indicated in the Clear Header.

7. Error Messages

 There are two kinds of error message built into the encapsulation
 protocol.  The first is used to report unknown flow identifiers seen
 by a Decapsulator and the second is for the forwarding of ICMP
 messages.
 When a Decapsulator is using the received Flow ID in an Encapsulation
 Header to forward a datagram to the next Decapsulator in a Flow, it
 is possible that the Flow ID may not be known.  For this case the
 Decapsulator will notify the previous Encapsulator that the Flow was
 not known so that the problem may be reported to the layer
 responsible for the programming of the Flow tables.  This is
 accomplished through an encapsulation error message.
 If an Encapsulator receives an ICMP messages regarding a given flow,
 this message should be forwarded backwards along the flow to the
 source Encapsulator.  This is accomplished by the second kind of
 error message.  The ICMP message will contain the Flow ID of the
 message which caused the error.  This Flow ID must be translated to
 the Flow ID relative to the Encapsulator to which the error message

Woodburn & Mills [Page 10] RFC 1241 Internet Encapsulation July 1991

 is sent.
 If an error occurs while sending any error message, no further error
 message are generated.

8. References

 [1]  J. Postel,  Internet  Control  Message  Protocol,  RFC  792,
      September 1981.
 [2]  J. Postel, Internet Protocol, RFC 791, September 1981.
 [3]  J. Postel, Transmission Control Protocol, RFC 793, September
      1981.
 [4]  ORWG, Inter-Domain Policy Routing Protocol Specification and
      Usage, Draft, August 1990

A. Packet Formats

 This section describes the packet formats for the encapsulation
 protocol.
      0               8              16              24            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Vers  |  HL   |  MT   |  RC   |            Checksum           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Flow ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Fig. A.1.  Encapsulation Protocol Header Example
     Vers      4 bits    The  version   number  of  the  encapsulation
                         protocol.     The  version  of  the  protocol
                         described by this document is 1.
     HL        4 bits    The  header   length  of   the  Encapsulation
                         Protocol Header in octets.
     MT        4 bits    The  message   type  of   the   Encapsulation
                         Protocol message.    A  data  message  has  a
                         message type  of 1.   An  error message has a
                         message type of 2.
     RC        4 bits    The reason code.  This field is unused in the
                         Data Message  and must have a value of 0.  In
                         the Error Message it contains the reason code
                         for the  Error Message.   Defined reason code

Woodburn & Mills [Page 11] RFC 1241 Internet Encapsulation July 1991

                         values are:
                              1 Unknown Flow ID
                              2 ICMP returned
     Checksum  16 bits   A   one's   complement   checksum   for   the
                         Encapsulation Protocol Header.  This field is
                         set to 0 upon calculation of the checksum and
                         is  filled   with  the  checksum  calculation
                         result before the data message is sent.
     Flow ID   32 bits   The Flow  ID as  seen by  the Decapsulator or
                         Encapsulator to  which this  message is being
                         sent.   In the  case of  an Unknown  Flow  ID
                         error, the Flow ID causing the error is used.

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

Woodburn & Mills [Page 12] RFC 1241 Internet Encapsulation July 1991

 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.

Woodburn & Mills [Page 13] RFC 1241 Internet Encapsulation July 1991

 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

Woodburn & Mills [Page 14] RFC 1241 Internet Encapsulation July 1991

 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.

Woodburn & Mills [Page 15] RFC 1241 Internet Encapsulation July 1991

                                    ++++++  Virtual Network A
                                    ******  Virtual Network B
                                         #  Encapsulator/Decapsulator
                                    ------  Common Routing Space
  1. ———– ————

/ \ / \

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

Woodburn & Mills [Page 16] RFC 1241 Internet Encapsulation July 1991

 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:  (703) 734-9000 or (703) 448-0210
 EMail:  woody@cseic.saic.com
 David L. Mills
 Electrical Engineering Department
 University of Delaware
 Newark, DE  19716
 Phone:  (302) 451-8247
 EMail:  mills@udel.edu

Woodburn & Mills [Page 17]

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