rfc1940.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

         generate a control packet with "Unimplemented source route
           protocol type"
       end if
       discard the packet
       return
     end if



     if the Hop Count field is greater than 0 begin
       decrement the Hop Count field
     end if
     if the Hop Count field is 0 begin
       if the packet is a data packet begin
         generate a control packet with "Hop Count Exceeded"
      end if
       discard the packet
       return
     end if


     if the packet is a data packet begin
       if the packet violates transit policy begin
         generate a control packet with "Transit Policy Violation"



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         discard the data packet
         return
       end if
     end if

     set mode to NONE
     set advanced to FALSE
     if Next Hop Ptr does not equal Source Route Length begin
       set NEXT_HOP to the next hop in the source route
       while mode equals NONE begin
         if NEXT_HOP is from network 127.0.0.0 begin
           set the Loose/Strict Source Route bit equal to
               the Loose/Strict Source Route Change bit
         else if NEXT_HOP is from network 128.0.0.0 begin
           set NEXT_DI to the least significant two octets of NEXT_HOP
           if NEXT_DI is not equal to LOCAL_DI begin
             set mode to DOMAIN
           end if
         else if NEXT_HOP does not equal an address assigned to the
           local router begin
           set mode to LOCAL
         end if
         if mode equals NONE begin
           set advanced to TRUE
           increment the Next Hop Pointer field
           if Next Hop Pointer equals Source Route Length begin
             set mode to COMPLETE
           else
             set NEXT_HOP to the next hop in the source route
           end if
         end if
       end while
     end if


     if mode equals DOMAIN begin
       set route to NONE
       if the source route is loose begin
         if not advanced begin
           find the route, if any, based on Prefix and Prefix Length
           if the route is an aggregate formed at the local router begin
             set route to NONE
           end if
         end if
         if route equals NONE begin
           select a BGP or IDRP route, if any, with a path that includes
             NEXT_DI and is not an aggregate formed at the local router
           if route equals NONE begin



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             if the packet is a data packet begin
              generate a control packet with "No Route Available"
             end if
             discard the packet
             return
           end if
           copy the NLRI from the route to the Prefix and Prefix Length
         end if
         if the route is an IDRP route begin
           set appropriate TOS in delivery header
         end if
         set NEXT_ROUTER from the route
       else
         set NEXT_ROUTER from the routing information for NEXT_DI
           using the D-FIB
         if route equals NONE begin
           if the packet is a data packet begin
             generate a control packet with "No Route Available"
           end if
           discard the packet
           return
         end if
         if NEXT_DI is not adjacent begin
           if the packet is a data packet begin
             generate a control packet with "Strict Source Route Failed"
           end if
           discard the packet
           return
         end if
       end if
       end if
     end if


     if mode equals LOCAL begin
       set NEXT_ROUTER equal to NEXT_HOP
       if the source route is strict and NEXT_ROUTER is not
         adjacent begin
         if the packet is a data packet begin
           generate a control packet with "Strict Source Route Failed"
         end if
         discard the packet
         return
       end if
     end if

     if mode equals LOCAL or mode equals DOMAIN begin
       set the destination address of the delivery header equal



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         to NEXT_ROUTER
       checksum the delivery header
       route packet to NEXT_ROUTER using normal IP forwarding
       return
     end if

     if the packet is a control packet begin
       discard the packet
     end if
     remove the delivery header and the SDRP Header
     if there is no normal IP route to the payload destination begin
       generate a control packet with "No Route Available"
       discard the data packet
       return
     end if
     forward the payload using normal IP forwarding
     if the probe bit is set begin
       generate a control packet with "Probe Completed"
     end if

5.2.2 Handling an SDRP control packet.

   An SDRP control packet is indicated by 0 in the Data packet/Control
   packet bit in the Flags field in the SDRP Header.

   If the Target Router field of the received SDRP packet contains an IP
   address that is assigned to the router that received this SDRP
   packet, then the router should use the information carried in the
   Notification Code field, the Source Route Identifier field and the
   information carried in the Payload field to update the status of its
   SDRP routes. Details of such procedures are described in Section 7.

   Otherwise, the router checks whether it can forward the packet to the
   router specified in the Target Router field by using the routing
   information present in its local FIB. If forwarding is possible then
   the local system sets the destination address of the delivery header
   to the address specified in the Target Router field, and hands the
   packet off for normal IP forwarding.  If normal IP forwarding is
   impossible then the packet may be forwarded in the same manner as an
   SDRP data packet (described below) but with the following exceptions.

      - Control packets are not subject to transit policies.
      - In no case should a control packet be generated in response to
        an error caused by a control packet.
      - If the source route is completely traversed and the packet still
        cannot be forwarded via normal IP routing, the packet should be
        silently dropped.




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5.2.3 Handling an SDRP data packet.

   An SDRP data packet is indicated by a one in the Data packet/Control
   packet bit in the Flags field in the SDRP Header.

   An SDRP data packet is forwarded by sending the packet along the
   source route in the SDRP Header.  When the source route is completely
   traversed and the packet has reached the destination domain, the
   payload may be removed from the data packet and forwarded normally.
   Further details are described below.

5.2.4 Checking the SDRP version number

   An SDRP packet that has a version number other than 1 should be
   discarded.  If the SDRP packet was a data packet, then a control
   packet with the Notification Code "Unimplemented SDRP version" should
   be generated as specified in section 6.

5.2.5 Checking the Source Route Protocol Type

   This document describes Source Route Protocol Type 1.  An SDRP router
   may support multiple Source Route Protocol Types; however an SDRP
   router is NOT required to support all defined Source Route Types.
   Any packet that has a Source Route Protocol Type which is not
   supported should be discarded.  If the SDRP packet was a data packet,
   then a control packet with the Notification Code "Unimplemented
   Source Route Protocol Type" should be generated as specified in
   section 6.

5.2.6 Decrementing and checking Hop Count

   If an SDRP packet is to be forwarded and the Hop Count field is non-
   zero, the Hop Count field should be decremented.  If the resulting
   value is zero and the packet was a data packet, then a control packet
   with the Notification Code "Hop Count Exceeded" should be generated
   and sent to the encapsulating router as specified in section 6, and
   the packet should be discarded.  If the resulting value is zero and
   the packet was a control packet, the packet should be discarded.  The
   payload of the control packet should carry the payload header
   followed by 64 bits of the payload data of the data packet.

5.2.7 Upholding transit policies

   It is not a goal of SDRP to create a security routing system.
   Therefore, we need to qualify our use of the term "upholding transit
   policy".  It is assumed that transit policies have the nature of a
   "gentleperson's agreement", and are upheld by all the participants.
   In other words, it is assumed that there will be no malicious



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   attempts to violate transit policies and that parties will rely on
   auditing and post facto detection of violations. When a security
   architecture is developed for IP or other network protocols then it
   may be applied to increase the assurance of transit policy
   enforcement. These issues are beyond the scope of this document.

   A router may examine any data packet to verify if it complies with
   local transit policies, as described in section 5.1.  If the
   verification fails, the router generates a control packet.  If the
   verification referred to only the contents of the SDRP header, then
   the payload field of the control packet should be empty. If the
   verification referred to both the contents of the SDRP header and the
   payload header, then the payload field of the control packet should
   carry the payload header.  If the verification referred to the
   transport protocol header, then the payload field of the control
   packet should carry the payload header and the transport header.

   The Notification Code field of the SDRP header in the control packet
   is set to Transit Policy Violation.  The procedures for constructing
   the rest of the SDRP Header of the control packet are specified in
   Section 6.

5.2.8 Partially traversed source routes

   If a router receives an SDRP packet with a partially traversed source
   route, it extracts the next hop of the source route from the Source
   Route field. The router locates the high-order byte of the
   appropriate hop by using the Next Hop Pointer field as a 32 bit word
   offset relative to the start of the Source Route field.  The next hop
   is always four octets long.  The following procedure is used to
   interpret the next hop.

   Syntactically, each element in the source route appears as an IP
   address.  There are three encodings for the next hop:

   a) The next hop is an address in network 127.0.0.0.  In this case,
   the Loose/Strict Source Route field is set equal to the Loose/Strict
   Source Route Change bit.  Then the Next Hop Pointer is incremented,
   the next hop is read from the Source Route field, and these three
   cases are examined again.

   b) The next hop is an address in network 128.0.0.0.  In this case,
   the DI of the next domain is extracted from the least significant two
   octets of the next hop.  If the extracted DI is the same as the DI of
   the local domain, then the Next Hop Pointer is incremented, the next
   hop is read from the Source Route field, and these three cases are
   examined again.  Otherwise, if the extracted DI is different from the
   DI of the local domain, the next hop is the extracted DI, and the



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   forwarding process may proceed.

   c) The next hop is any other IP address.  If the next hop is equal to
   any IP address assigned to the local router, the Next Hop Pointer is
   incremented, the next hop is read from the Source Route field, and
   these three cases examined again.  Otherwise, the next hop is the IP
   address of the next router in the source route and the forwarding
   process may proceed.

   The above procedure for interpreting the next hop in the source route
   finishes when the next hop is either a router other than the local
   router or an encoded DI that is not the local DI or a completed
   source route.

   If upon termination of this procedure the source route is completely
   traversed, see section 5.2.9.

5.2.8.1 Finding a route to the next hop

   If the next hop is not a DI, then the destination address in the
   delivery header is replaced by the next hop address and the resulting
   packet can then be forwarded using normal IP forwarding.  Otherwise,
   a DI was extracted from the next hop in the source route, and the
   following procedure is used to find a route to the next domain.

   Given the DI of the next domain, the router next consults its D-FIB.
   If no entry exists in the D-FIB for the next domain, then the packet
   should be discarded.  If the packet was a data packet, a control
   message with Notification Code "No Route Available" should be
   generated as specified in Section 6. No other actions are necessary.

   If there is a D-FIB entry, the router next examines the SDRP header
   to determine if the packet specified a strict source route.  If so,
   and the next domain is not adjacent to the local domain, then a
   control packet with the Notification Code "Strict Source Route
   Failed" should be generated, as specified in section 6, and the
   original packet should be discarded.  No other actions are necessary.

   If source route is loose, then BGP or IDRP information must be used
   to insure that there is no loop in reaching the next hop.  If the
   Next Hop Pointer was incremented when determining the next hop, then
   the router must select a BGP or IDRP route with a path that includes
   the extracted DI, and the NLRI for this route is copied into the
   Prefix Length and Prefix fields.

   Otherwise, the Next Hop Pointer was not incremented, and the router
   should use the information carried in the Prefix and Prefix Length as
   an index into its BGP or IDRP routing table.  If it finds a matching



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   route then it must select the corresponding D-FIB entry.  If the
   route was formed locally by aggregation, then the router must consult
   its D-FIB and select any route with a path that includes the
   extracted DI.  The NLRI for this route should be copied into the
   Prefix Length and Prefix fields.

   In either case, the D-FIB entry includes the IP address of the next
   SDRP-speaking router to which the SDRP packet should be routed.  The
   destination address in the delivery header is replaced by this
   address.  The resulting packet can then be forwarded using normal IP
   forwarding.