rfc1940.txt

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               bit is set to 1, it indicates that the next hop is a
               Strict Source Route Hop.  If this bit is set to 0, it
               indicates that the next hop is a Loose Source Route.

            Probe Indicator (bit 5)

               The Probe Indicator is used by the originator of the
               route to request verification of the route's feasibility
               (see Sections 4 and 7.1).  If this bit is set to 1, it
               indicates that the originator is probing the route.  This
               bit should always be set to 0 for control packets.

      Hop Count (1 octet)

         The Hop Count field carries the maximum number of routers an
         SDRP data packet may traverse. It is decremented by 1 as an
         SDRP data packet traverses a router which forwards the packet
         using SDRP forwarding. Once the Hop Count field reaches the
         value of 0, the router should discard the data packet and
         generate a control packet (see Section 5.2.6).  A router that
         receives a packet with a Hop Count value of 0 should discard
         the data packet, and generate a control packet (see Section
         5.2.6).

      Source Route Protocol Type (1 octet)

         The Source Route Protocol Type fields indicates the type of
         information that appears in the source route.  The value 1 in
         this field indicates that the contents of the source route are
         as described in this document and indicates an Explicit Source



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         Route.  The value 2 in this field indicates a Route Setup.  The
         syntax of the source route for this value is identical to a
         value of 1, but also has additional semantics which are defined
         in other documents.

      Payload Protocol Type (1 octet)

         The Payload Protocol Type field indicates the protocol type of
         the payload.  If the payload is an IP datagram, then this field
         should contain the value 1.

         Note that this Payload Protocol Type is not the same as the IP
         protocol type[5,7].

      Source Route Identifier (4 octets)

         The BR  that originates the SDRP packet should insert a 32 bit
         value in this field which will serve as an identifier for the
         source route.  This value needs to be  unique  only in the
         context of the originating BR.

      Target Router (4 octets)

         This field is meaningful only in control packets.

         The Target Router field contains one of the IP addresses of the
         router that originated the SDRP packet that triggered the
         control packet to be returned.

      Prefix (4 octets)

         The Prefix field contains an IP address prefix.  Only the
         number of bits specified in the Prefix Length are significant.
         The Prefix field is used to prevent routing loops when using
         BGP or IDRP to route to the next AS in a loose source route
         (see Section 4).

      Prefix Length (1 octet)

         The Prefix Length field indicates the length in bits of the IP
         address prefix.  A length of zero indicates a prefix that
         matches all IP addresses.

            Notification Code (1 octet)

               This field is only meaningful in control packets.  In
               data packets, this field is transmitted as zero, and
               should be ignored on receipt.



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               This document defines the following values for the
               Notification Code:

               1 - No Route Available

               2 - Strict Source Route Failed

               3 - Transit Policy Violation

               4 - Hop Count Exceeded

               5 - Probe Completed

               6 - Unimplemented SDRP version

               7 - Unimplemented Source Route Protocol Type

               8 - Setup Request Rejected

      Source Route Length (1 octet)

         The Source Route Length field indicates the length in 32 bit
         words of the domain level source route carried in the SDRP
         Header.

      Next Hop Pointer (1 octet)

         The Next Hop Pointer field indicates the offset of the high-
         order byte of the next hop along the route that the packet has
         to be forwarded.  This offset is relative to the start of the
         Source Route field; so if the value of the Next Hop Pointer
         field equals the value of the Source Route Length field, then
         the entire source route has been completely traversed.  All
         other source routes are said to be incompletely traversed.

      Source Route (variable)

         The components of the source route are syntactically IP
         addresses.

         An IP address from network 128.0.0.0 is used to encode a next
         hop that is a domain.  The least significant two octets contain
         the DI, which is an Internet Autonomous System number.








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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      128      .       0       |             D. I.             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         An IP address from the network 127.0.0.0 is used to encode
         characteristics of the source route.  The least significant
         three octets are used as a Source Route Change field.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      127      |          Source     Route     Change          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Source Route Change (3 octets)

            Loose/Strict Source Route Change (bit 1)

               The Loose/Strict Source Route Change bit reflects a new
               value of the Loose/Strict Source Route bit in the SDRP
               header.  The value of the Loose/Strict Source Route
               Change bit is copied into the Loose/Strict Source Route
               bit in the SDRP header when a Source Route Change field
               is encountered in processing an SDRP packet.

            The rest of the Source Route Change field is transmitted as
            zero, and should be ignored on receipt.

      Payload (variable)

         The Payload field carries the datagram originated by the end-
         system within the domain that constructed the SDRP packet. The
         Payload field forms the data portion of the SDRP packet.  In a
         control packet this field may be empty or may carry the payload
         header of the packet that triggered the control message (see
         5.2.5).  Note that there is no padding between the Source Route
         and the Payload, and that the Payload may start at any
         arbitrary octet boundary.










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4.  Originating SDRP Data packets

   This document assumes that a router that originates SDRP packets is
   preconfigured with a set of SDRP routes.  Procedures for constructing
   these routes are outside the scope of this document.  SDRP packet
   forwarding may be deployed initially without additional routing
   protocol support.

   An application on a source host generates packets that must be
   delivered to a given destination.  The packet traverses the Internet
   by following normal hop-by-hop routing information.  An intermediate
   router in the path between the source host and the destination host
   may decide to forward some of these packets via SDRP.

   When this router receives an IP datagram, the router uses the
   information in the datagram and the local criteria to determine
   whether the datagram should be forwarded along a particular SDRP
   route.  Associated with each set of criteria is a set of one or more
   SDRP routes that should be used to route matching packets.  The exact
   nature of the criteria is a local matter.  The only restrictions this
   document places on the applicability of SDRP routes is that an IP
   datagram that contains a strict source route should not be forwarded
   along an SDRP route, that SDRP encapsulation should never be applied
   to an SDRP packet, and that if SDRP is used with inter-domain routes,
   the destination domain must also run SDRP.

   If the router decides to forward a datagram along a particular SDRP
   route, the router constructs the SDRP packet by placing the original
   datagram into the Payload field of the SDRP packet and constructing
   the SDRP header based on the selected SDRP route.  The Next Hop
   pointer is set to 0 (the first entry in the Source Route field of the
   SDRP packet).  The value of the Time To Live field in the payload
   header should be copied into the Hop Count field of the SDRP header.

   Even if we assume that interior routing is loop free, it is possible,
   either due to the state of inter-domain routing or due to other SDRP
   routers, that a domain level source route that does not terminate
   with the intended destination domain may lead a packet into a routing
   loop.  Originating SDRP routers that wish to insure that this does
   not occur should include a final domain level hop of the
   destination's domain, i.e. specify the SDRP route as <DI1, DI2, DI3>
   instead of <DI1, DI2>, if the destination host is in domain DI3.  The
   means for determining the DI of the destination domain is outside of
   the scope of this document.

   Similarly, when using SDRP for interior routing, it is possible that
   the source route does not coincide with IGP routing.  In this case,
   one means of preventing a loop is to specify the last hop router's IP



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   address as the last address within the source route.  The
   encapsulating router can do this by specifying the source route to
   reach destination host IP3 as <IP1, IP2, IP3> instead of <IP1, IP2>.

   The source address field in the delivery header should contain an IP
   address of the router. The value of the Don't Fragment flag of the
   delivery header is copied from the Don't Fragment flag of the payload
   header.  The value of the Type Of Service field in the delivery
   header is copied from the Type Of Service field in the payload
   header.  If the payload header contains an IP security option, that
   option is replicated as an option in the delivery header.  All other
   IP options in the payload header must be ignored.

   If the SDRP route that is used is learned from IDRP, then the TOS
   corresponding to this route is copied into the TOS field in the
   delivery header.

   The resulting SDRP packet is then forwarded as described in Section
   5.2.2.

   If the encapsulating router decides to forward a datagram along a
   particular SDRP route that has an MTU smaller than the length of the
   datagram, then if the payload header has the Don't Fragment flag set
   to 1, the router should generate an ICMP Destination Unreachable
   message with a code meaning "fragmentation needed and DF set" in
   accordance with [6].  The ICMP message must be sent to the original
   source host.  The router should then discard the original datagram.

   If a router has learned an MTU for a particular SDRP route, either
   via ICMP messages or via configuration information, and it determines
   that an SDRP packet must be fragmented before transmission, then it
   first calculates the the effective MTU seen by the payload packet.
   If the effective MTU is greater than or equal to 512 bytes, the
   router SHOULD first fragment the payload packet using normal IP
   fragmentation.  SDRP packets are then constructed for each fragment,
   as describe above.  Otherwise, the router should first form the SDRP
   packet, and then fragment it.

   A router may use locally originated  SDRP packets to verify the
   feasibility of its SDRP routes. To do this the router sets the value
   of the Probe Indicator field in the SDRP packet to 1.  Receipt of an
   SDRP control packet by the originating router with the "Probe